Common term for tenant-based keys Assume the following schema:

ID  | TENANT | TENANT_CUSTOM_ID
346 | 101    | 1  
347 | 102    | 1  
348 | 101    | 2  
349 | 101    | 3  
350 | 101    | 4  
351 | 102    | 2  
352 | 103    | 1


What is another name (or the proper classification) for TENANT_CUSTOM_ID? Perhaps something along the lines of a surrogate key? The purpose of this key is to make each tenant have their own numbering system instead of knowing that their records are mixed with all other tenants on the same tables.
A natural key (AKA business key or domain key) is a key that is used to identify information in the business domain.

A surrogate is a key that is "unseen", internal to the database, meaningless and not used in the business domain.

The distinction that really matters is whether the attribute in question is exposed to users / consumers and therefore serves some function and acquires meaning as part of a real world process.

It seems that there is already too much loose terminology and folklore associated with keys. I don't think it would be helpful to have another term for the kind of key you are referring to here.