Nodes are points in activity diagrams at which an activity takes places. Edges are connecting lines between individual nodes. So-called tokens migrate across these connecting lines. These tokens indicate the status of what is happening. Let’s go into more detail:
An initial node depicts the beginning of an activity. The initial node is the beginning of the process. If there is more than one initial node then some of the affected parallel branches start the process together. If an initial node is not present then all nodes that have no incoming edges will be interpreted as starting points.
An activity final node ends an activity. After all actions have been completed then the process flow of the activity ends. This point is depicted by the final node. An activity diagram can contain any number of final nodes and it’s possible for it to be absent in ongoing processes.
Decision nodes depict alternative workflows. Alternative workflows are generally attached to conditions.
A synchronization is a reunion of branches. Subsequent actions are only carried out if only one alternative path has been carried out.
For non-subsequent workflows, i.e. in cases in which the order of the editing steps is irrelevant, the UML offers the notation elements called splitting and synchronization. A splitting node braches a control flow into multiple paths while a synchronization reunites the control flows.