Hi Bruce,
There is very little information out there on this because it is a very specific area of expertise, not related to BW at all really.
If you are asking how delta is identified, it is based on form bundle timestamp. You can refer to table DFMCA_RETURN, field AETSP.
The standard form bundle extractor extracts the current version of data only. You cannot load different versions, neither extractor structure nor generic FM FMCA_FPF_FORM_DATA_EXTRACT_BI are prepared for data version extraction, it is not supported, even extraction of form versions (DFMCA_FORM_VERS).
I would recommmend reading through all notes in the component area IS-PS-CA and XX-PROJ-FI-CA, this is the component area related to the Public Sector Contract Accounting extractor. I doubt you will get any information on this topic in this content area, maybe try on the Business Content and Extractors pages...
http://scn.sap.com/community/data-warehousing/business-content-and-extractors/content
Hope this helps,
Colm