You can set which fields in the attendees profile should be mandatory, optional or hidden. This can be adjusted by an account with 'owner' rights under organization settings in the dashboard, read more here.
Phone number, e-mail address, first name and last name are always mandatory fields. If you set another field to 'mandatory', this will be shown with a *, so that the visitor can see which fields are mandatory and optional. Everything you set to 'hidden' will not be shown.
Settings in the dashboard:
Requested attendee information: