The GeoDCAT-AP API is a proof-of-concept of the implementation of GeoDCAT-AP in an OGC CSW (Catalog Service for the Web), re-using the standard CSW interface, and supporting in addition HTTP content negotiation.
More precisely, GeoDCAT-API uses the standard CSW parameters outputSchema and outputFormat to determine, respectively, (a) the GeoDCAT-AP profile to be used (core or extended), and (b) the RDF serialisation to be returned.
The document containing the ISO 19139 records to be transformed is specified by a GeoDCAT-API-specific parameter src, which is not part of the CSW interface.
The API uses the GeoDCAT-AP XSLT to transform ISO 19139 records into GeoDCAT-AP. As such, the API works both on static files including the records, and on the CSW output of a GetRecords or GetRecordById request.