It would rebuild based on the date of threads, registration, etc. For example:
- User ID 1: Legitimate user joined on March 20, 2014
- User IDs 2-9: Spam bots joined on March 21, 2014
- User ID 10: Legitimate user joined on March 22, 2014
Rebuilding user IDs would make user ID 10 user ID 2 and any future registrations will start at user ID 3.
Or if IDs got changed when importing:
- User ID 1: Joined on March 25, 2014
- User ID 2: Joined on March 20, 2014
User ID 2 is actually user 1, but got changed around when importing. Rebuilding IDs would give user ID 1 to user 1 again.