Not strictly true!..
Mixes can be set up to overlap in any order.
In this test there is 1 layer per mix layer1-mix1, layer2-mix2, layer3-mix3, layer4-mix4 (so as to identify the mixes)
In this example the mix 'stack' as shown on screen is as follows:
Mix3, Mix1, Mix2, Mix4
(see jpg)
However, it is true that when you restart catalyst, the priority will default to 1,2,3,4 1 at bottom, 4 at top... But mixes can operate in any priority if manually set-up.
Simon