These three measurements describe different parts of one carved face. Keeping them separate makes quotations clearer and reduces production mistakes.
Face thickness
The starting thickness of the material before carving. For example, an 18 mm face begins as a full 18 mm board or timber layer.
Relief depth
The maximum amount removed from the original face at the deepest carved point. If the CNC removes 8 mm from an 18 mm face, the maximum relief depth is 8 mm.
Remaining material
The material left behind the deepest carved area. In that same example: 18 mm starting face minus 8 mm maximum removal leaves 10 mm behind the carving.
Example: Starting face: 18 mm · deepest CNC removal: 8 mm · remaining material: 10 mm. This is a planning reference; the router-ready toolpath still depends on the final artwork, dimensions, cutter and material.