Decomposition must be consistent!
The explicit declaration of contained instances defines a tree aggregate structure of instances.
The decomposition of an instance within one MSC must refer to an MSC within the decomposed instance (kind).
In the foils we have that the instance ACSystem of the MSC UserAccess is decomposed using an MSC AC_UserAccess contained in the MSC document defining ACSystem. This is consistent.
Further requirements to consistency is given in the next slide.