From UML2.1.2: 7.3.12 Dependency (from Dependencies):

'• client: NamedElement [1..*]
The element(s) dependent on the supplier element(s). In some cases (such as a Trace Abstraction) the assignment of direction (that is, the designation of the client element) is at the discretion of the modeler, and is a stipulation. Subsets DirectedRelationship::source.'