See UML Superstructure Specification, v2.0, figure 263, p. 340.

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 Joinpseudo-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, v2.0, p. 471) 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."