Anything that was missing when the application was first received, or anything they feel they need to see to make a complete assessment of the applicant (or the sponsor) I've seen all of the following: requests for a copy of a passport that wasn't included upfront because the applicant didn't yet have a passport when the ap was submitted; missing criminal clearances; proof of a medical exam, or they can't locate the medical results (even when you provided proof of the medical with the ap), and then you have to track that down; (more) proof that an overseas sponsor intends to return to Canada; if there was a past criminal offence - all of the documentation regarding that (court records, sentencing report, etc.); more documentation of the relationship - but in that case there's usually an interview scheduled and you're asked to bring the documentation with you to the interview. There can also be a demand for further medical exams and, now apparently, even psychiatric assessments and drug testing!