Cutting only down not sideways?

If I want to cut say only one face of an item I am not able to use the Area command but have to make a dummy part and pocket. I am thinking is there a command I am missing or is this presently the way it works….shown the dummy part and pocket with the face I was wanting to trim.

workspace_FEMALE_DUMMY_PART (1).kmz (210.0 KB)

workspace_TOP_TUBE_1_INDEX_FIXT (2).kmz (141.3 KB)

angled face was what I wanted to machine.

Part is done so no rush here