Loop client channels
This action starts an unnamed loop of all the channels the currently selected client is joined to.
For each channel, the Client > On channel clients loop condition is triggered, with that one channel selected, so you can use it in any action, condition or expression that refers to the selected channel. The client you ran the loop for is also selected.
Once the loop is over, the Client > On channel clients loop finished condition 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 "Client > On client's joined channels loop" and "Client > On client's joined channels loop finished".
Loop contents during interactive conditions
In Relay, during a channel join request, the joining channel will not be in the loop for a joining client. During a leave request, the channel will be in the loop.
In Blue, the channel may or may not be in the loop, depending on the response mode the server is set to by the Enable/disable conditions > On join channel request / On leave channel request.