The Subtract from Selection command creates a selection in the current layer from the Alpha Channel. Opaque pixels are fully selected, transparent pixels are unselected, and translucent pixels are partially selected.This selection is subtracted from the existing selection. The Alpha channel itself is not changed.

7.40.2. Example

Figure 15.106.  Applying “Subtract from Selection”

Non-transparent pixels of the active layer have been subtracted from the existing rectangular selection.