In his latest video, the 8-Bit Guy goes over how the Commodore REUs for the 64 and 128 work with the Commodore hardware.
While it’s not mentioned in the video the Color 64 BBS program supported the 17XX RAM Expansion Units from Commodore. You need a unit must have at least 256k of storage, so either the 1764 or 1750, to be usable with Color 64 BBS. Color 64 uses the REU by installing RAMDOS in order to simulate a Commodore disk drive. The version of RAMDOS included with Color 64 supports a RUE expanded with up to 2 megabytes of memory.