The Initial element is used by Activity and StateMachine diagrams. In Activity diagrams, it defines the start of a flow when an Activity is invoked. With StateMachines, the Initial element is a pseudostate used to denote the default state of a Composite State; there can be one Initial vertex in each Region of the Composite State.
This simple example shows the start of a flow to receive an order.
See the OMG Unified Modeling Language specification, (v2.5.1, Figure 12.97, p.378.)
The activity flow is completed by a Final or Flow Final node.
- Moving a diagram generally does not affect the location of elements in Packages; if you move a diagram out of one Package into another, all the elements in the diagram remain in the original Package
However, Initial elements are used only within one diagram, have no meaning outside that diagram, and are never re-used in any other diagram; therefore, if you move a diagram containing these elements, they are moved to the new parent Package with the diagram
OMG UML Specification:
Initial in Activity Diagrams
The OMG Unified Modeling Language specification, (v2.5.1, p.387) also states:
An InitialNode is a ControlNode that acts as a starting point for executing an Activity. An Activity may have more than one InitialNode. If an Activity has more than one InitialNode, then invoking the Activity starts multiple concurrent control flows, one for each InitialNode.
An InitialNode shall not have any incoming ActivityEdges, which means the InitialNodes owned by an Activity will always be enabled when the Activity begins execution and a single control token is placed on each such InitialNode when Activity execution starts. The outgoing ActivityEdges of an InitialNode must all be ControlFlows. The control token placed on an InitialNode is offered concurrently on all outgoing ControlFlows.
Initial in State Machine Diagrams
The OMG Unified Modeling Language specification, (v2.5.1, p.312) states:
An initial Pseudostate represents a starting point for a Region; that is, it is the point from which execution of its contained behavior commences when the Region is entered via default activation. It is the source for at most one Transition, which may have an associated effect Behavior, but not an associated trigger or guard. There can be at most one initial Vertex in a Region.