PLM — Empty Interfaces in PLM Services Specification dtc/04-05-05

In several places in the model, there are classes that have both realization and association relationships with interfaces. An example is the Specification_expression class that holds association and realization relationships with the Specification_operand_select interface. The PLM Adopted Specification (OMG document dtc/04-05-05) in its section 7.7.12 describes the PLM Configuration Management Package. In it, figure 104 illustrates the association relationship between the Specification_expression and the Specification_operand_select. Later in section 7.7.12.42 the interface Specification_operand_select is defined as: " This empty interface is defined to provide a placeholder for the following classes: Specification_expression and Specification"

The realization relationhsip between a class and an interface is perfectly clear to me. It is the association relationships that "provides a placeholder" that I cannot understand. Does it mean that such a placeholder is an instance of an element that implements the interface?

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