XXDP is the diagnostics system for PDP-11 computers. There are two versions available: XXDP 2.2 (I say 2.2 only because the image is called xxdp22.rl02. I have never seen anything in its contents that would indicate a version.) and XXDP 2.5. XXDP 2.2 is more primitive than 2.5. Both are found as RL02 disk images here. Google for "xxdp" to find several different sets of personal notes. Pete Turnbull's reference is very good.
Definitive information on XXDP and the diagnostics for the PDP-11 are quite difficult to come by. Apparently this was also true within DEC because recently this document has come to light. It appears to be a reference for the most common PDP-11 diagnostics, but from its format does not seem to have originally been an official manual.
CHMDXC0 XXDP+ DX MONITOR
BOOTED VIA UNIT 0
ENTER DATE (DD-MMM-YY):
RESTART ADDR: 132010
THIS IS XXDP+ TYPE "H" OR "H/L" FOR HELP.
Type H for a list of commands, sub-commands and modifiers. The most important secret I discovered is that to make a disk bootable you must LOAD the monitor and then SAVM it. There are some .CMD files on the disk that are setup for creating different media. They seem to require some setup, but you can get an idea of the command sequences. UPD2 is the most useful tool. It is a more capable command interpreter than the base monitor.
The files on this image do not seem to be complete. For example, the XXBLD.CCC command file attempts to run UPD2.BIC, the chainable version of UPD2 (?), but there is not such file in the image.
This image does not seem to be complete either. It does not have CMD files.