Subteams
A subteam is a slice of one team: some of its servers, and the people who work on them. If your team runs several servers and not everybody should see all of them, subteams are how you split them up.
Access runs through subteams. Adding somebody to a team no longer gives them anything on its own - they see a server only when a subteam they are in covers it. Somebody in no subteam sees nothing at all.
Making one
Open Team Manager → Subteams. Name the subteam after the servers it covers, then tick which of your team's servers it includes and which of your people work on them. A subteam only starts granting anything once it has at least one of each; until then the card says so.
Somebody can be in as many subteams as you like, and they see everything those subteams add up to.
Who can change them
Only the Super Team Manager who owns the team. Team Managers can see subteams but not edit them, because a Team Manager can be inside one and editing would let them widen their own access.
The Super Team Manager is never limited by subteams. They always see every server on their own teams, since they are the one doing the assigning.
Taking access away
Removing somebody from a subteam removes their access to those servers at once. So does removing them from the team, or unlinking the server from the team - you do not have to tidy up the subteam as well.
The Subteams page counts how many of your people currently have no access and names them, so an unassigned moderator is not left wondering why their server list is empty.