Two programs in a run unit can reference common data in the following circumstances:
- The data content of an external data record can be referenced from any
program provided that program has described that data record.
- If a program is contained within another program, both programs can refer
to data possessing the global attribute either in the containing program or
in any program that directly or indirectly contains the containing program.
- A parameter passed by reference can be shared between the calling program
and the called program.