When you chanage the active layer, it changes the visible stack-up. The active layer is on top (except it's below pins, pads, silk and soldermask). The layer is a circle, so the stack-up depends on what's on top. e.g. layer 1 is active, stack-up is 1, 2, 3, 4, 5, 6, 7, 8 e.g. layer 3 is active, stack-up is 3, 4, 5, 6, 7, 8, 1, 2