From what I can find this info isn't explicitly provided by the institute. Annoyingly.

smithsonian_unit_codes_and_descriptions.json

{
      "AAA": {
        "name": "Archives of American Art",
        "description": "The Archives of American Art collects and preserves primary sources documenting the history of the visual arts in the United States."
      },
      "AAG": {
        "name": "Anacostia Museum and Center for African American History and Culture",
        "description": "Focuses on the history and culture of African Americans and the African diaspora, highlighting community and cultural contributions."
      },
      "ACAH": {
        "name": "Smithsonian American Art Museum, Cultural Artifacts",
        "description": "Preserves and displays cultural artifacts significant to American art history and heritage."
      },
      "ACM": {
        "name": "Anacostia Museum & Center for African American History and Culture",
        "description": "Dedicated to preserving, interpreting, and presenting African American history and culture, particularly from a community perspective."
      },
      "ACMA": {
        "name": "African American Cultural and Musical Archives",
        "description": "Archives focused on the collection and preservation of African American cultural and musical heritage."
      },
      "CFCHFOLKLIFE": {
        "name": "Center for Folklife and Cultural Heritage",
        "description": "Promotes the understanding and continuity of diverse contemporary cultures through research, documentation, and educational programs."
      },
      "CHNDM": {
        "name": "Cooper Hewitt, National Design Museum",
        "description": "Dedicated to historic and contemporary design, showcasing the impact of design on daily life."
      },
      "EEPA": {
        "name": "Eliot Elisofon Photographic Archives",
        "description": "Houses extensive photographic collections that document African cultures and their histories."
      },
      "FBR": {
        "name": "Freer Gallery of Art and Arthur M. Sackler Gallery, Records",
        "description": "Archives and records of the Freer and Sackler Galleries, focusing on Asian art and culture."
      },
      "FSA": {
        "name": "Freer Gallery of Art and Arthur M. Sackler Gallery Archives",
        "description": "Preserves and provides access to materials related to the study and appreciation of Asian art."
      },
      "FSG": {
        "name": "Freer - Sackler Galleries",
        "description": "Home to the Smithsonian's Asian art collections, hosting exhibitions and educational programs."
      },
      "HAC": {
        "name": "Hirshhorn Art Collections",
        "description": "Maintains the collections of modern and contemporary art housed at the Hirshhorn Museum."
      },
      "HMSG": {
        "name": "Hirshhorn Museum and Sculpture Garden",
        "description": "Focuses on modern and contemporary art, providing a national platform for artists."
      },
      "HSFA": {
        "name": "Human Studies Film Archives",
        "description": "Collects and preserves ethnographic film and related materials documenting human cultures."
      },
      "NAA": {
        "name": "National Anthropological Archives",
        "description": "Houses extensive collections of anthropological research and cultural documentation."
      },
      "NASM": {
        "name": "National Air and Space Museum",
        "description": "Displays the history of aviation and space exploration, showcasing significant artifacts and exhibits."
      },
      "NASMAC": {
        "name": "National Air and Space Museum Archives Center",
        "description": "Preserves documents, photographs, and other materials related to the history of aviation and space exploration."
      },
      "NMAAHC": {
        "name": "National Museum of African American History and Culture",
        "description": "Dedicated to documenting African American life, history, and culture, featuring artifacts and stories of resilience."
      },
      "NMAH": {
        "name": "National Museum of American History",
        "description": "Collects and exhibits artifacts that reflect the social, political, cultural, scientific, and military history of the United States."
      },
      "NMAI": {
        "name": "National Museum of the American Indian",
        "description": "Dedicated to the life, languages, literature, history, and arts of the Native Americans of the Western Hemisphere."
      },
      "NMAIA": {
        "name": "National Museum of the American Indian Archives",
        "description": "Houses archival materials documenting the history and cultures of Native American peoples."
      },
      "NMAfA": {
        "name": "National Museum of African Art",
        "description": "Features traditional and contemporary African art, aiming to foster a deeper understanding of African cultures."
      },
      "NMNHANTHRO": {
        "name": "National Museum of Natural History, Department of Anthropology",
        "description": "Conducts research and houses collections related to human cultural and biological diversity."
      },
      "NMNHBIRDS": {
        "name": "National Museum of Natural History, Division of Birds",
        "description": "Research and collections related to avian species from around the world."
      },
      "NMNHBOTANY": {
        "name": "National Museum of Natural History, Department of Botany",
        "description": "Focuses on the study and preservation of plant biodiversity."
      },
      "NMNHEDUCATION": {
        "name": "National Museum of Natural History, Office of Education",
        "description": "Develops educational programs and resources related to natural history."
      },
      "NMNHENTO": {
        "name": "National Museum of Natural History, Department of Entomology",
        "description": "Research and collections related to insects and their ecological impact."
      },
      "NMNHFISHES": {
        "name": "National Museum of Natural History, Division of Fishes",
        "description": "Studies fish species and maintains collections for scientific research."
      },
      "NMNHHERPS": {
        "name": "National Museum of Natural History, Division of Amphibians and Reptiles",
        "description": "Research and collections related to amphibian and reptile species."
      },
      "NMNHINV": {
        "name": "National Museum of Natural History, Department of Invertebrate Zoology",
        "description": "Studies invertebrate species and maintains extensive collections for scientific purposes."
      },
      "NMNHMAMMALS": {
        "name": "National Museum of Natural History, Division of Mammals",
        "description": "Research and collections focused on mammalian species."
      },
      "NMNHMINSCI": {
        "name": "National Museum of Natural History, Department of Mineral Sciences",
        "description": "Researches minerals, gems, rocks, and meteorites, maintaining significant collections."
      },
      "NMNHPALEO": {
        "name": "National Museum of Natural History, Department of Paleobiology",
        "description": "Studies fossils and ancient life forms, maintaining extensive paleontological collections."
      },
      "NPG": {
        "name": "National Portrait Gallery",
        "description": "Displays portraits of individuals who have made significant contributions to American history and culture."
      },
      "NPM": {
        "name": "National Postal Museum",
        "description": "Explores the history of the postal service and philately, showcasing related artifacts."
      },
      "NPMA": {
        "name": "National Postal Museum Archives",
        "description": "Preserves archival materials related to the history of the postal service and philately."
      },
      "NZP": {
        "name": "National Zoological Park",
        "description": "A zoological park focused on animal care, science, education, and sustainability."
      },
      "OCIO_DPO3D": {
        "name": "Office of the Chief Information Officer, Digital Preservation Office 3D",
        "description": "Specializes in the digital preservation of three-dimensional artifacts and records."
      },
      "OFEO-SG": {
        "name": "Office of Facilities Engineering and Operations, Smithsonian Gardens",
        "description": "Manages and maintains Smithsonian gardens, contributing to the institution's aesthetic and educational mission."
      },
      "SAAM": {
        "name": "Smithsonian American Art Museum",
        "description": "Collects and exhibits American art, encompassing all regions and cultures of the United States."
      },
      "SAAMPAIK": {
        "name": "Smithsonian American Art Museum, Nam June Paik Archive",
        "description": "Dedicated to the works and legacy of video art pioneer Nam June Paik."
      },
      "SI": {
        "name": "Smithsonian Institution Archives",
        "description": "Preserves the history of the Smithsonian Institution through documentation and archival materials."
      },
      "SIA": {
        "name": "Smithsonian Institution Archives",
        "description": "Maintains records and materials documenting the history and activities of the Smithsonian Institution."
      },
      "SIL": {
        "name": "Smithsonian Libraries",
        "description": "Provides library and research services across the Smithsonian's various museums and research centers."
      },
      "SILAF": {
        "name": "Smithsonian Libraries, African Art Library",
        "description": "Focuses on providing resources related to African art and culture."
      },

      "SILNMAHTL": {
        "name": "Smithsonian Libraries, National Museum of American History, Trade Literature",
        "description": "Houses trade literature collections that document American industrial and commercial history."
      },
      "SILSRO": {
        "name": "Smithsonian Libraries, Smithsonian Research Online",
        "description": "Provides access to research outputs and publications from Smithsonian scholars."
      },
      "SLA_SRO": {
        "name": "Smithsonian Libraries, Smithsonian Research Online",
        "description": "Supports the research activities of Smithsonian Institution staff and provides public access to a wide range of scholarly publications."
      }
    }