The Initial element is used by Activity and State Machine diagrams. In Activity diagrams, it defines the start of a flow when an Activity is invoked. With State Machines, the Initial element is a pseudo-state 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 UML Superstructure Specification, v2.1.1, 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:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 537) states:
An initial pseudostate represents a default vertex that is the source for a single transition to the default state of a composite state. There can be at most one initial vertex in a region. The outgoing transition from the initial vertex may have a behavior, but not a trigger or guard.
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 378) also states:
An initial node is a control node at which flow starts when the activity is invoked.