Loop clients
This action starts an unnamed loop of all the clients in the currently selected channel.
For each client, a Channel > On client channels loop event is triggered, with that one client selected, , so you can use it in any action, condition or expression that refers to the selected client. The channel you ran the loop for is also selected.
Once the loop is over, the Channel > On clients channels loop finished event is triggered once.
All those loop events happen instantly. Read more about how loops work here.
In Blue Server, these conditions are renamed to be clearer, as "Channel > On channel's clients loop" and "Channel > On channel's clients loop finished".
In Relay Server, there is no "Loop clients (with loop name)" action, despite there being conditions for such an action. However, it is available in Blue Server, and you can view it here.