Understanding the Contact Universe in Query Interface for Medical Lists

Hi all, quick question: what is the universe of contacts that we query when we build a list through the query interface? For instance, if I were to build a list of medical professionals with a nurse degree in the US, from what database(s) would Clay have built the universe that I’d be querying? The reason I’m asking is to confirm that their query would return the totality of the targeted contacts I’m looking for vs. just a subset. Hope the question makes sense.