The CPCEMU disc image format (conventional extension
.DSK) is conventionally used for images of Amstrad CPC, Amstrad PCW, and Spectrum +3 floppy disks.
It is thus the de facto disc image format for most 3" floppy disks. (There's probably no technical reason it couldn't image other disk types, but typically it doesn't.)
Such disk images will often (but not always) contain a CP/M file system.