A DocumentRequest, composed of ResourceRequests, and a few other values, is submitted to the API. The API then uses that information to fetch assets associated with each Resource and interleaves said assets according to the assembly strategy chosen. It also builds interlinking between assets within the document as appropriate and then translates the assets into an HTML document. Finally the API generates a PDF, ePub, or Docx from the HTML document if requested.
Each Resource is composed of lang_code, resource_type, and book_code.
There are hundreds of lang_codes, several different resource_types, and at least 66 book_codes.
There are a few different assembly strategies available some of which also have different layout sub-strategies.
In this way, the API can produce multi-language, multi-book, multi-resource PDFs assembled according to the resources, assembly strategy, and layout combinations chosen.