Fuzzy Grouping Transformation - is it possible to specify the source/target data sets being compared.

I'm working on a project for a client that uses the Fuzzy Grouping Transformation component to identify potential duplicate records in the system. It appears that you can define a query to use as the source of records look through... but as far as I can tell, the FGT uses that query to then look through the rest of the database and I need to restrict that to look at just a subset of the data in the database... sometimes that subset will be the whole database, other times it won't be (more often than not it'll be a true subset) ... Does anyone have any experience using the FGT? Is there a way to limit what the logic looks at? or do I need to look at everything first, then remove from the results data that isn't in the subset?