You are not able to associate a specification with a Declarative Expression rule. However, as you rightly suggest, you want to capture the business rule into a specification. The question is where?

For a decision rule specification, it is more confusing and tricky, because of the many ways in which an application can use decision rules.

If your decision is not used to direct flow processing, like a declarative expression, then you can add the description of the behavior to the appropriate assignment or flow action specification.