An instance of an MSC may be decomposed in form of a sub Message Sequence Chart (sub MSC), thus allowing a top-down specification.

A sub MSC essentially has a structure analogous to an MSC. It is distinguished from the MSC by the keyword submsc . Characteristic for a sub MSC is its relation to a decomposed instance containing the keyword decomposed and having the same name as the sub MSC. The relation is provided by the messages connected to the exterior of the sub MSC and the corresponding messages sent and consumed by the decomposed instance.

[In MSC-96 there will be an extension to the decomposition phrase such that any MSC can be specified as the sub MSC.