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 alt , par , loop , opt and exc operators are described in definition of operator . The seq operator denotes the weak sequencing operation where only events on the same instance are ordered.
The subst operation is a substitution of concepts inside the referenced MSC. Message names are substituted by message names, instance names by instance names and MSC names by MSC names.