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The Join element is used by Activity and StateMachine diagrams. The example illustrates a Join transition between Activities. With respect to StateMachine diagrams, a Join pseudosate indicates multiple States concurrently transitioning into the Join and onto a single State. Unlike Choice or Junction pseudostates, Joins must not have triggers or guards. This diagram demonstrates a Fork pseudostate dividing into two concurrent Regions, which then return to the End State via the Join.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 538) states:
Join vertices serve to merge several transitions emanating from source vertices in different orthogonal regions. The transitions entering a join vertex cannot have guards or triggers.