In Clarity 16.4.1, a new feature has been added to allow a Lookup to be associated to the attribute in the "Query" section of a Data Provider (more information here).
When attempting to associate a Lookup, many appear to be missing.
Clarity 16.4.1
This is due to a limitation of the design. Currently only Lookups that are API enabled, or are DWH enabled will display in the Lookup list returned. For this to be true, the Lookup must be associated to an Attribute on an Object. Once that is completed the Lookup can be API enabled or included in the DWH.
This functionality will be expanded in a future release of Clarity to allow other Lookups to be included.