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The idea is to allow callback data to be custom formatted by the plugin. Essentially the plugin writer would provide a method that could be called if it exists to format the response in a custom way.
Use case: RunDeck
RunDeck requires data to be in a certain format. See here under File Formats. The idea is that, in lieu of using Ephemerals to store the XML, we would be able to push updated resource data to RunDeck so it can consume it (say, new entries to the Host primitives)