Unwrap to Empty Trim
DW in edit mode
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.
One face or multiple faces, with a Trim Sheet Material, whose sheet has an Empty Trim defined.
ALT: Also remove seam marks of boundary edges and select them
CTRL: Actually Unwrap the selection instead of using UV Reset