As part of th exAPI, you define your own activities, activity types, verbs, extensions and attachments as part of statements. All of these require identifiers that should resolve to a URL that provides all of the spec-required metadata for each. There should be a a place where everyone in the community can find identifiers that have already been defined. It creates problems if there are many different identifiers for a verb, activity, attachment or extension that mean the exact same thing. This makes xAPI data very hard to work with. Data normalization is hard, and we all need to use a registry in order to work with xAPI data well.
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