Require Roster Selection
This is a checkbox at the site-level under Site Details in the section "Attendance & Meal Count Settings".
Only Sponsor Admin users can edit this checkbox.

Select this box to require centers or providers to select a roster when adding a new participant.
If this box is unchecked and the user doesn't select a roster when creating a participant, the participant will be added to whichever roster is designated as the "Default Roster"