Rules Regulating the Scope of Program Names
The following rules apply to referencing a program-name of a program that is contained within another program. For this discussion, we will say that Program-A directly contains Program-B and Program-C, Program-C directly contains Program-D and Program-F, and Program-D directly contains Program-E.

If Program-D does not possess the COMMON attribute, then Program-D can only be referenced by the program that directly contains Program-D, that is, Program-C.
If Program-D does possess the COMMON attribute, then Program-D can be referenced by Program-C since it contains Program-D, and by any programs contained in Program-C except for Program-D and programs contained in Program-D. In other words, if Program-D possesses the COMMON attribute, Program-D can be referenced in Program-C and Program-F, but not by statements in Program-E, Program-A, Program-B, or Program-D.