See UML Superstructure Specification, v2.1.1, figure 12.103, p. 384.
The Join element is used by Activity and State Machine diagrams. The above example illustrates a Join transition between Activities. With respect to State Machine diagrams, a Join pseudo-state indicates multiple States concurrently transitioning into the Join and onto a single State. Unlike Choice or Junction pseudo-states, Joins must not have triggers or guards. The following diagram demonstrates a Fork pseudo-state 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.