The edX course numbers are "invented" for RIT and edX reference purposes.
Using the example of PM9002x, "PM" refers to Project Management (the subject). The "9000" series designates to us that the course is an advanced level course (in comparison to a 100-level course, which is introductory). Anything 500 and above is considered advanced level. The "2" is the number of the course in the series (i.e. PM9003x is the third course in the series). The x is to designate it as an edX course (not an RIT main course).