I have 3 assumptions actually, just assumptions:
1. It's a CIT0520 form that asks for supplementary documents (normally passport request), but I haven't seen one being issued by mail during this pandemic (based on the forum), so this might be unlikely.
2. Maybe it's some kind of form that asks you to update your location? - Again, never seen this being requested since address change can be finished online.
3. the full-blown CIT0171 residence questionnaire - they might have doubt about your physical presence so they need you to complete it, this also diverts your process into non-routine processing. If this was the one that was issued, this is not a good news, RQ can extend the process by months if not years.