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R56 / 1c:Mc-1
Stem is decorated by zero or one Semantic Expression
Semantic Expression decorates zero, one or many Stem
Every Stem (point where a Connector meets the face of a Node) has meaning.
For example, in an association relationship of a class model, each Stem will signify a multiplicity and conditionality such as 1 mult, Mc mult, etc. In a generalization relationship a Stem touching a subclass indicates, well, the subclass semantic.
That's what is meant when we say that a drawn Stem indicates (or signals) some Stem Signfication. And we define these significations such that there is always exactly one, by policy.
A given Stem Signification, (1 mult) for example, can appear on multiple Stems on a given Diagram or none at all.