HP 3000 Manuals

HP System Dictionary XL Gen. Ref. Vol. 1

Linking Restrictions 

Note that you can link entities only to entities, and you can link
relationships only to relationships, and only from a local domain version
to a common domain version which must be linked together first.  Further,
you can make links only between occurrences of the same type.  For
example, you cannot link an entity of type RECORD to an entity of type
FILE, nor can you link a relationship of type DATABASE contains DATASET
to a relationship of RECORD contains ELEMENT.

MPE/iX 5.0 Documentation