The parser will not offer "visible" or "accessible" features. It is an add-on for the PDF parser which is shipped with FPDI and extend its features to parse compressed cross-reference tables and streams.
It will also allow FPDI to import pages of faulty or corrupted PDF documents (not all, but there are fallbacks implemented for common issues).
Since version 2.1 the add-on also supports importing pages of encrypted or protected PDF documents.
The usage of FPDI is not changed nor extended in any way.