What is the right way to define an area inside a board that has to be removed by milling by the PCB manufacturer. Maybe i can use Insert->Area->Polygon for it and change the area-type to 'Board' but i am not shure if this is the right way . How does my PCB manufacturer know that he has to remove the inside part of the board ?
You add a cut-out, but first you specify it is a board cut-out by selecting a segment of the board, then use Insert Cut-out of whatever shape you require. This ensures the cut-out is part of the board outline for the manufacturer