Amendments have to be in scope. Scope is "what the bill is about”. The scope is determined by the bill’s existing clauses and schedules.
You can submit an amendment about something that is not already in the bill, but any new topic must be reasonably close to the bill's existing content.
If you think your amendment might be outside the scope of the bill, contact the Public Bill Office as soon as possible (email PBOHoC@parliament.uk or phone 3251).