Edit in place not allowed on some mechanical components?

Why do some mechanical components give the error message 'Cannot REFEDIT anonymous blocks' when I try and edit in place using either REFEDIT or by double clicking?
For example, FR-02 cannot be edited in place, however FR-01 can.
Also I have no idea why the icons in the browser are different to each other. Can somebody point me to a help resource that explains mechanical browser icons?


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