Ok, just to clarify my meaning:
In link static data in SSMS / SQL Src Control, it doesn't give the option to specify the fields to use for the comparison key, and just uses any PK defined in the db itself.
In SQL Data Compare itself it gives the option to specify the fields.
In my particular case there is a PK used for update and delete operations, but the real unique compound keys are 3 string fields.
Is there any way I can get SSC to use these