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 UML Superstructure Specification, v2.1.1, Figure 12.97, p.378.
The activity flow is completed by a Final or Flow Final node.
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.