From UML2.1.2:

Description

Realization is a specialized abstraction relationship between two sets of model elements, one representing a specification (the supplier) and the other represents an implementation of the latter (the client). Realization can be used to model stepwise refinement, optimizations, transformations, templates, model synthesis, framework composition, etc.

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Semantics
A Realization signifies that the client set of elements are an implementation of the supplier set, which serves as the specification. The meaning of ‘implementation’ is not strictly defined, but rather implies a more refined or elaborate form in respect to a certain modeling context. It is possible to specify a mapping between the specification and implementation elements, although it is not necessarily computable.

Notation

A Realization dependency is shown as a dashed line with a triangular arrowhead at the end that corresponds to the realized element. Figure 7.73 illustrates an example in which the Business class is realized by a combination of Owner and Employee classes.'

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