If login attempts to a user's account exceeds this threshold, that account will be locked for a pre-configured amount of time

Normal - Users can register from the /register page.
Admin Approval - User registrations are placed in an approval queue for administrators.
Admin Approval for IPs - Normal for new users, Admin Approval for IP addresses that already have an account.
Invite Only - Users can invite others from the users page.
Admin Invite Only - Only administrators can invite others from users and admin/manage/users pages.
No registration - No user registration.

0 for no restriction. Admins get infinite invitations
Only applicable for "Invite Only"