Mainly used to generate CSL citations out of BibliographyItem lists that are transformed into HTML fragments with specific CSL styles (styles are mirrored from here https://github.com/citation-style-language/styles). But it can also parse various bibliography formats (.bib, .ris, papers-citations-xml, citeproc+json) into BibliographyItems as well as perform various utility functionalities.
A similar implementation of creating citations can be found here https://github.com/Zettlr/Zettlr/blob/dd2cbda72d9499edae6a1d95fb977794bde1ed4e/source/app/service-providers/citeproc-provider.ts that might be useful for understanding the problem context.