Division

 As a simplistic but useful reduction: A composition is a list of one or more divisions.  A division is a list of measures that "begins" after some indefinite period of silence and "ends" with the last measure in  the list with the same general expectation of another indefinite period of silence there after.  If music has any form of syntax it seems necessarily true that the Division is a syntactically complete and consistent unit even if that Division is part of a larger whole whose syntactic completeness and consistency depends on those parts.

The MeasureList not only contains measures, it contains context data that applies to and completes the "meaning" of its measures.  It is represented by the Yellow rectangle in the figure to the left labeled Master Channel Set which contains all the Channels in a Division.  Note that the measure objects also have context data that applies to the their ChannelBars.