Ram on an x99. What kind? X99 platforms have the ability to run full quad-channel ram, which the regular boards cannot, they can only run 2x sets of dual channel. But to run quad will take both sides of the socket. All things being equal, single channel is slow, dual channel faster and quad channel fastest throughput, how much so depending on how the ram is used, what's using it, even which slot can affect it (according to mobo vendors recommendations).
(Ddr3/ddr4 x79/x99 same configurations)
Also depends on the ram itself. Having 4 sticks in the same kit doesn't necessarily mean it's quad channel capable, it's usually 2x sets of dual channel rated. This makes a difference because the bios chip on the ram only allows for dual channel no matter what slots you use physically.
For normal, 2x sticks of dual channel with big air, you'd use C1/D1.
For normal 4x sticks of dual channel with big air C1C2/D1D2, populate both channels on same socket side.
With an aio, this can change as the socket is no longer height restricted, so you can use B1/D1, or with 4x sticks B1B2/D1D2 etc. For quad channel ram it'd be 4x sticks, A1/B1/D1/C1 etc.