It is used to merge several incoming transitions from different regions into one outgoing transition.
A join belongs to a region.
A join is shown as a short bold bar.
Symbol in context
The join is reached even if not all the regions have an outgoing transition to the join. Only the two transitions incoming the join have to be enabled. This is the case if simple states 'Simple state 1' and 'Simple state 3' are finished.
A join has at least two incoming transitions and exactly one outgoing transition.
Different incoming transitions of a join cannot leave the same region.
The outgoing transition of a join cannot have a guard or triggers.
All incoming transitions of a join have a state as a source.
Interpretation of and deviation from the UML specification
There is no interpretation of and deviation from the UML specification for a state machine's join.