Remaining below-rim camp space, opened to everyone on the 1st of the month after the lottery (e.g. Feb 1 for May start dates).
Grand Canyon's own backcountry permit page describes an Early Access Lottery: applications run on Recreation.gov from the 16th of the month to 5 p.m. MST on the 1st, four months before the desired start month, with applicants notified on the 2nd. On the 1st of the following month everyone can reserve remaining camp space. We watch both moments.
Updated , checked by hand against the target's own page.
Watched from nps.gov, the target's own public page — not a reservation system we poll.