It depends if users have already registered on your system:
- If there are no attendees on your system, everyone will be forced to fill in mandatory fields upon registering.
- If you already have attendees on your system that registered before new fields were made mandatory, they will not need to fill the new fields in until they edit their profile. To enforce any new mandatory fields, it is suggested you either fill these in yourself per attendee or email everyone to ensure their profiles are up to date.
Learn more about Custom Fields by visiting this article.