Often Views, defined by an aggregation pipeline, are used to filter out certain fields, certain records and obfuscate parts of certain values to enable users with a specific restricted role to only see a subset of 'less sensitive' data from a collection. Views can be assigned to a role declaratively, but in some use cases it is also useful to allow the aggregation pipeline logic to be able to access the context of the current session's roles (e.g. $$ROLES) or user id (e.g. $$USER) to be able to make some programmatic decisions of what to show in the view based on current user & role (i.e. to perform programmatic role based access control). In some examples, this aggregation pipeline may even want to use $$ROLES or $$USER to then perform a $lookup on a different 'metadata' collection, to influence what it decides to return as the output of the view.