When you're at maximum capacity, any new recruit will land in the Waiting Room. Mother Base will of course try to optimizeassign waiting staff assignment by replacing oldto whichever team they're best suited to. If that team is full, Mother Base will replace any less suited staff with newthe waiting one. Those who are leastIf no less suited for astaff can be found, Mother Base will try to assign the waiting staff to their next best suited team until all teams have been tried. Naturally, if any of your staff dies on a mission or needs medical attention, a replacement will be found in the Waiting Room.
Any staff that has been replaced will not be reassigned to one whereanother team, even if they cancould replace someone even less suitedelse on that team, at least not immediately. It's likely they will be assigned to ita new team the next time Mother Base chooses to assign them, which normally happens when new staff arrives, or land inleave the Waiting RoomBrig or Sick Bay, or old staff die. You will need to manually trigger an Auto Assign if there aren't anyyou do not wish to wait.
However, for someSome of your staff, Mother Base won't ever reassign them. Those are staff members with unique skills. If you have a low-rank Russian interpreter, he will remain in your Support team unless you specifically assign him somewhere else. If you happen to have 2 Russian interpreters, the lower-ranked one will be reassigned, while the other will remain in your Support team. This isn't limited to interpreter skills and also extends to R&D-relevant ones, like Bionics Specialist and more. These people won't Auto Assign, even if you ask for it. You'll have to manually assign them to a specific team.
People who just left the Brig will be assigned to the teams best suited to them, but they won't replace anyone, even if they'd be better suited to it. Instead they'll land in the Waiting Room until you choose to assign or Auto Assign them.1 People who are currently away on missions obviously cannot change their team until they've returned, and some of your staff are locked to specific teams until the story demands otherwise, such as...
Huey (R&D) and Quiet (Brig or Sick Bay)
Finally, people who you have manually assigned to a team will never be auto assigned by Mother Base or replaced by more competent staff, unless you manually assign or trigger an Auto Assign. Staff that are under a Direct Contract can only be assigned manually, and won't everwill never automatically change their team, even if you ask for an Auto Assign.
What happens when the Waiting Room is full, along with the rest of Mother Base? Some of the people waiting inside will automatically be dismissed. Who exactly is chosen, I don't know. It could be the oldest recruit, or it could be the lowest ranked one, or it could also be chosen at random.
1: There are time when people who leave the Brig will replace less suited staff in full teams, and times when that doesn't happen until It is therefore best to occasionally clean up your Waiting Room so you specifically request an Auto Assign. I haven't yet been abledon't lose some of your better staff to pin down when it happens and when it doesn'tE-rank ones, so I've decidedespecially if you're about to strike this point out for the time beingexpand your base.