Almost every week of the year, there's a PGA Tour event. On the Friday before the next tournament starts on Thursday (except with the WGC-Dell Technologies Match Play), the PGA Tour tournament entry deadline comes and goes to lock in a field.
The PGA Tour entry deadline is the Friday before the tournament week at 5 p.m. local time (before open qualifying, in the case of an open event).
However, sometimes a PGA Tour event is unfolding in a different time zone or part of the world when the entry deadline passes. This means players competing in a PGA Tour event when the entry deadline passes area allowed to enter a tournament on a different deadline. For players competing in a PGA Tour event when the deadline comes, they have 30 minutes after play completes to decide to enter a tournament or not.
Almost every PGA Tour event has different criteria to qualify for an event field. If a player doesn't match those criteria, they can't get in that field. Some tournaments exclusively use the PGA Tour's priority order for determining a field. Most do not. They use some combination of their own criteria and then the PGA Tour's priority order, giving exemptions to past champions, highly-ranked players in the Official World Golf Ranking and other exemptions for various reasons.
The entry list for each PGA Tour event, then, is ranked and subject to change. For open events and invitationals outside the World Golf Championships and Masters Tournament, if a player withdraws, the next player on the alternate list gets into a tournament.
However, if a player with higher priority joins a field at the entry deadline, they can push an otherwise committed player to the alternate list.
The same thing can happen in an open PGA Tour event -- one with Monday qualifiers -- when a non-member player scores a top-10 finish. Under PGA Tour rules, non-members who earn a top-10 finish (outside of World Golf Championships events) automatically earn a berth into the next open event on the Tour schedule. That means an otherwise committed player could be kicked to the alternate list to make room for a player based on their high finish in a prior event.
For the World Golf Championships events, there isn't really an entry deadline; a player qualifies or they don't under the criteria laid out ahead of the tournament. World Golf Championships event don't have alternates, so the maximum field is whoever qualifies. Any players who withdraw -- and, why would then when there's no cut and free money and FedEx Cup points? -- are not replaced in the field.