rmarkdown and bookdown offer two different methods for managing citations and bibliographic references in a document.
The default setting for rmarkdown and bookdown (and pandoc itself)
is to use a pandoc helper program (specifically, a “filter”) called pandoc-citeproc,
which follows the specifications of the Citation Style Language (CSL) and obtains specific formatting instructions from one of the huge number of available CSL style files (default is chicago-author-date.csl
).
In some circumstances, users might prefer to use either natbib (based on bibtex) or biblatex as a “citation package” instead.