Adding a fillet in a subtraction operation:

It's possible to add a fillet when computing the subtraction operation. This fillet will be computed on the intersection edges created by the subtraction operation:

The interest of computing a fillet at the same time as the subtraction operation is that the fillet computation is linked to the subtraction operation rather than to edges. If you change one or all objects the fillet can still be computed on the intersection edges from the subtraction operation.