What is the difference between Join and Merge in Unified Modeling Language Activity Diagram. Give an example to understand more clearly.
Join Node (see reference 1):
Join node is a control node that has multiple incoming edges and one outgoing edge and is used to synchronize incoming concurrent flows. Join nodes are introduced to support parallelism in activities.
Merge Node (see reference 2):
Merge node is a control node that brings together multiple incoming alternate flows to accept single outgoing flow. There is no joining of tokens. Merge should not be used to synchronize concurrent flows.
For example in below diagram:
A decision is used after a fork, the two flows coming out of the decision need to be merged into one before going to a join.
Why?: Otherwise, the join will wait for both flows.
So, Activity 2 and Activity 3 are our alternate flows and only one of which will arrive. And they are not synchronize incoming.
However, the Concurrent_Activity and result of decision between Activity 1 and Activity 2 (that merged into one output) are synchronize incoming concurrent flows. The join waits for both to perform and continue.
To elaborate on @Gholamali-Irani's answer: Activity diagrams are derived from Petri nets. In short you have to imagine a single "token" that starts at the one initial start point (that fat dot). The token travels along a path until it vanishes in one of those (circled dot) final terminals (or as UML allows in actions which have no outgoing path). The fork nodes will multiply that single token into as many tokens as it has outgoing paths (UML also has an implicit fork for actions with multiple outgoing paths which are not guarded). So these multiple tokens travel independently until they either vanish like described above or they reach a join (or action) with multiple ingoing paths. Here the token waits until all paths are fed with a single token. These multiple tokens are then merged into a single one which travels on as usual.
With this ruleset you can model any complex concurrent network.
来源：https://stackoverflow.com/questions/48136302/join-and-merge-in-activity-diagram