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Remark: Other feedbacks from the academic federations running categories for a significant time would be enlightening. 

The DFN-AAI Experience (DE)

The Identity Federation operated by the German Research and Education Network (DFN) introduced Entity Categories (both for SPs and IdPs) in 2012 in order to support so-called "Virtual Sub-Federations". The setup is based on a whitelist maintained by a specific project or community and which is hooked up with the metadata registry. The project-specific EC is only available for entities listed on such a whitelist - a nightly check removes the EC automatically if an entity disappears from the respective whitelist.

The Greek Federation experience (GR)

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