I can see I haven't explained it well enough :-)
Database A, master schema, has dictionary tables such as 'ApplicationName' and 'CustomerType'.
Databases B, C and D are entirely different schemas, however also contain applications and customers of types which need to be validated against a master list.
If I create the 'ApplicationName' table in both databases, the data can get out of synch. I *do* need the tables in each of the A, B, C and D schemas to be validated against these master lists, and can't create FKs if 'ApplicationName' sits outside of these in its own schema, *or* if 'ApplicationName' exists in just one of the schemas.
Do you see?