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The documentation here is not actual. (only available for nb 6.1) The better is the test case: http://uml.netbeans.org/qa/nb65/SanityTests/UMLSanityTestSpecification.html#Create_Diagram_From_Selected_Elements it should be linked in old article, or created a new with a link to this one.
Documentation for the UML plugin has not been supported and, hence, updated since the 6.1 release. Unfortunately, no one from the docs team can be assigned to fix this issue. Closing the issue as won't fix.