Ordliste
Machining allowance — definition
Extra material cast onto faces and into holes that will later be machined.
Casting gives coarser tolerances than machining. If a face needs a tighter tolerance than the casting process holds, it has to be machined — and there has to be material there to take off.
The allowance must be large enough that the whole face cleans up even when the casting tolerance runs the unfavourable way, and small enough that it does not waste cutting time and metal.
This is why it helps to state in the enquiry which faces will be machined. Then we put allowance exactly there, and nowhere else.