Pretty much everything is involved!
The relationship is of course one. Then there are all of the things you sent in, do the birth certs check out, is the passport OK and not forged? Are all of the documents and replies consistent (e.g. is the name and DOB always the same)? Are there medical complications, do they have a communicable disease? There are also 'background checks' - are they who they say they are? Do the submitted photos match the photos on the medical and passport? Do they have a criminal record they have not disclosed? Are they on any terrorist 'watch lists'? Are there any records elsewhere on visa fraud or visa overstaying? Lots and lots of stuff.
But if none of this applies to you and your PA, then really, don't worry.