The LABEL list should contain the branch on which it is defined
This info is missing from both the label list and the trigger.
The only way to get the branch name from a label is to perform a two step operation: Check on which changeset it is set and then look for that changeset branch (in the changeset list).
Labels are decorated with the branch they belong to.
Göran Wallgren commented
A trigger is a script callback hook that you can install on the server side, see https://www.plasticscm.com/documentation/triggers/plastic-scm-version-control-triggers-guide.shtml
Triggers seem to be available for creation of the Label itself, but not for the action of applying (or moving) a Label to a Changeset.
I can confirm that the Branch info is nowadays present as a column in the Labels view.
Isn't this implemented by now? I can see the branches in the label list.
Also, what do you mean by "Trigger"?