hi kelkel,
there are lots of docs that can be requested, like any document or form that may have been sent, but asked to re-send or some new forms to be filled for more info that the embassy needs, or it could be some additional proofs or queries that need to be taken care of, or proof of relationship in case what was submitted wiht the application seems insufficient to the IO, it could be info on re-establishment plans after PR is granted or proof that sponsor is in canada or some docs like PCC or meds if have expired ...
so it can be any thing that the IO feels required further on to process the file.
hope this was helpful
mrs. canada