Unwrap to Empty Trim
shortcut DW in edit mode
The Unwrap to Empty Trim tool is used to remove Trim Detail, so faces with with a Trim Sheet Material, that were unwrapped to a specific part of a sheet.
Keep in mind
The tool doesn't actually remove faces, UVs or anything else. It just moves the UVs of the selection to the designated Empty Trim of a sheet.
Selection
One face or multiple faces, with a Trim Sheet Material, whose sheet has an Empty Trim defined.
Modes
ALT: Also remove seam marks of boundary edges and select them
CTRL: Actually Unwrap the selection instead of using UV Reset