I discussed this over breakfast this morning with a friend who owns an independent repair shop. M-B makes what is required within the realm of OBD-II regulations easily available to most "off the shelf" code readers.
But if a code it is not
required in the OBD-II specification it may
be a M-B proprietary code. In that case you will still see
the code, or at least part of it, but will not get a definition on most code readers. The manufacture of his professional test set charges $1,500 annually
for a M-B subscription to access all the possible OBD-II code definitions.
Since he doesn't see many M-B's in his shop he doesn't have it installed.
I found a website that shows which codes are manufacture proprietary. See Here
and scroll down for how codes are broken down at the high level.