As before: that's for extra M.2 storage - in the past, it was just a cheeky way of a manufacturer implying that the motherboard has more onboard M.2 storage than it actually has (probably only has 2 slots, but was advertised as 4)....
That said.... and in Asus' defence: I think with some of the Z690 boards, I think they might be doing it due to concerns with PCIe5 and not wanting to commit to putting a PCIe5 M.2 socket onto the motherboard that causes problems later.... safer to stick with PCIe4 on the motherboard and have the "PCIe5 ready" option available, in case people need to choose between storage and graphics for the PCIe5 channel.