The Unwrap Trim tool is used to unwrap a selection in edit mode to a specific Trim in a Trim Sheet.
Once unwrapped you can always re-unwrap the faces using Unwrap Trim.
You can also adjust them without unwrapping.
To remove detail created with Unwrap Trim, you can use the Unwrap Trim to Empty tool.
Keep in mind
Unwrap Trim will automatically create a Trim Sheet Material, if necessary and will act like the Init Trim Sheet Material tool in object mode.
If you don't like this behavior, setup and assign your sheet materials in advance using that tool.
One face or multiple connected ones.
ALT: Also unwrap the inverted selection to the sheet's Empty Trim
CTRL: Unwrap using CONFORMAL method, instead of ANGLE_BASED