Look at specs for any X399 board and they will say something like "Supports Quad Channel DDR4 3600+(OC) & ECC UDIMM Memory" or "8 x DIMM, Max. 128GB, DDR4 3600(O.C.)... MHz ECC and non-ECC, Un-buffered Memory". Both omit buffered/registered DIMMs.

AM4 boards have similar verbiage.