From the (Flight Software Branch C Coding Standard - 582-2000-005 - Version 1.1 - 11/29/05)
§2.1 UNITS (1) Code shall be structured as units, or as stand-alone header files.
(2) A unit shall consist of a single header file (.h) and one or more body (.c) files. Collectively the header and body files are referred to as the source files.
(3) A unit header file shall contain all pertinent information required by a client unit. A unit’s client needs to access only the header file in order to use the unit.
(4) The unit header file shall contain #include statements for all other headers required by the unit header. This lets clients use a unit by including a single header file.