An Occurrence relationship indicates that a Collaboration represents a classifier, in a Composite Structure diagram. An Occurrence connector is drawn from the Collaboration to the classifier. 

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OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.0, p. 161) states:

"A dashed arrow with a stick arrowhead may also be used to show that a collaboration represents a classifier, optionally labelled with the keyword «occurrence»."