Channel A has 1 ea, DDR3 slot and 1 DDR4 slot, same with channel B - it's basically stressing to you that you can't run both DDR3 and DDR4 at the same time or in otherwords can't populate all the slots, they had hybrid mobos like this in the 775 socket when the change from DDR2 to DDR3 came and many ruined mobos and CPU trying to run both, trying to cram DDR3 or 2 into the opposite sockets, etc