Parts are run-time instances of Classes or Interfaces. Multiplicity can be specified for a Part, using the notation:

[x{...y}]

where x specifies the initial or set amount of instances when the composite structure is created, and y indicates the maximum amount of instances at any time.

Parts are used to express composite structures, or modeling patterns that can be invoked by various objects to accomplish a specific purpose. When illustrating the composition of structures, Parts can be embedded as properties of other Parts. When embedded as properties, Parts can be bordered by a solid outline, indicating the surrounding Part owns the Part by composition. Alternatively, a dashed outline indicates that the property is referenced and used by the surrounding Part, but is not composed within it. 

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OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.0, p. 12) states:

"An element representing a set of instances that are owned by a containing classifier instance or role of a classifier. (See role.) Parts may be joined by attached connectors and specify configurations of linked instances to be created within an instance of the containing classifier."

The OMG UML specification (UML Superstructure Specification, v2.0, p. 14) also states:

A role is "the named set of features defined over a collection of entities participating in a particular context. Collaboration: The named set of behaviors possessed by a class or part participating in a particular context. Part: a subset of a particular class which exhibits a subset of features possessed by the class Associations: A synonym for association end often referring to a subset of classifier instances that are participating in the association."