The aggregate hierarchy of MSC-2000
“Commutative decomposition” was a term coined already as a methodological rule associated with MSC-96 (see Haugen in SDL Forum 97). With MSC-2000 this methodological rule has become a static requirement.
Intuitively it means that whether decomposition is followed by referencing, or referencing followed by decomposition this should end up in the same behavior.
In our example the MSC UserAccess has decomposition of instance ACSystemand reference to the utility EstablishAccess.
Following decomposition first, we arrive at AC_UserAccess defined in MSC document ACSystem. AC_UserAccess refers the utility AC_EstablishAccess.
Following referencing from UserAccess we come to EstablishAccess where the ACSystem instance is decomposed in AC_EstablishAccess. We conclude that the decomposition and referencing are consistent.