There are basically three ways to use <jsp:attribute>:
1) in concert with <jsp:element>, to dynamically build content
2) as a way to specify (declared) attribute values for custom tags
3) to set dynamic attributes
This last option (3) is not recognized by the JSP editor (as of 5.1.2 and 6.0.2).
Relevant quote from the JSP 2.0 spec, section 5.10:
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The behavior of the <jsp:attribute> standard action varies depending on the type of attribute being specified, as follows:
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If the custom action accepts dynamic attributes (Section JSP.7.1.8), and the name of the attribute is not one explicitly indicated for the tag, then the container will evaluate the body of <jsp:attribute> and assign the computed value to the attribute using the dynamic attribute machinery. Since the type of the attribute is unknown and the body of <jsp:attribute> evaluates to a String, the container must pass in an instance of String.
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To summarize:
-if <jsp:attribute> is used to set a custom action attribute that is not declared, the editor should accept this if the custom action supports dynamic attributes.