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To enable user-based metrics the user has to be identified with a consistent identifier. The identifier is wrapped in a context object and must be consistent for all events concerning a single user:
The identifier should be anonymized, i.e. a generated id without any connection to personal data. This has to be done on a best efforts basis. We know that making this work 100% of the time is impossible because users can clear their device cache or change devices or something else to break the consistency.
User-based metrics are metrics that actually require to tie an event stream to a certain user.
One example is the metric about order lines that are generated through 8SELECT content.
To know that specific products where bought because a user interacted with that product via the 8SELECT content all those events need to be tied to that specific user, i.e. need a user id in the context. Otherwise it is not possible to match order events with view or interact events.