Depends on the version. MP/M specifies user areas of 0-15, but CP/M 2.x specifies that they're from 0 to 31. And QX-10 Valdocs used user area 96 to store its files. The enforcement in CP/M and MP/M is a simple mask in the Get?Set User Area system call.
Why area and not number? Because CP/M is essentially single-user. All the user area does is add another "character" to the file name in the directory to further qualify the name. Nothing very sophisticated at all. MP/M, on the other hand, is multi-user and does associate a user console with a user area.
As far as interchangeability among systems, there's a lot more to interchange that involves how the diskettes are formatted and laid out. I suspect that there were somewhere around 1000 variations on disk formatting and layout for CP/M.
As far as I'm aware, the only "standard" CP/M format was 8", single-density, single-sided, 26 sectors of 128 bytes at a 6:1 logical interleave with the first two tracks reserved for system use--the original "A1" format.