MSC references may not only refer to a single MSC, but also to MSC reference expressions. MSC reference expressions are textual MSC expressions constructed from the operators alt, par, seq, loop, opt, exc and subst, and MSC references.
The actual gates of the MSC reference may connect to corresponding constructs in the enclosing MSC. By corresponding constructs we mean that an actual message gate may connect to another actual message gate or to an instance or to a message gate definition of the enclosing MSC. Furthermore an actual order gate may connect to another actual order gate, or an orderable event or an order gate definition.