While the X-MEM addresses the lower ROM, ROMs 0-6 and 8-31, the Y-MEM addresses the ROMs from 32 to 63.
The X-MEM allows to access 512 KB RAM using the port &7Fxx. But the Y-MEM addresses another 512 KB using the port &7Exx.
Using the X-MEM and the Y-MEM together provides 1 MB of expansion RAM and 1 MB of Flash-ROM for the CPC or Plus computers.
The Y-MEM (RAM and ROM) is fully supported by FutureOS