A 'Usage' is a Class diagram relationship in which one element requires another element for its full implementation or operation. The example diagram shows that the Class Order requires the Class LineItem for its full implementation.
The 'Usage' relationship is a subtype of a 'Dependency' relationship.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.38) states:
A Usage is a Dependency in which one NamedElement requires another NamedElement (or set of NamedElements) for its full implementation or operation. The Usage does not specify how the client uses the supplier other than the fact that the supplier is used by the definition or implementation of the client.