This seems to be the source to CP/M-68K v1.2

The .IMG files are 12 disk images. Standard IBM 3740 format

The d<n>u<m> directories are the files extracted from each disk. Disk n, User m

The UserN directories are the files for a particular user from ALL the disks gathered together. It's what you'd get if you copied the 12 original disks onto a large enough hard disk or similar.