How do the Finder search parameters work in a REST API Call

Summary:

I am curious about how the finder functionality works during a REST API call and would like an official response on it. Having worked with them for awhile it seems the search logic is fuzzy when my expectation is that it should not be. For example we are using the get external payee rest api to lookup an external payee based on the supplier id and the business unit ( we have an internal rule where there is only one supplier site/payee) per business unit.

To this we use the following search

?finder=ExternalPayeeSearch;OrganizationName=ET BU,PayeePartyIdentifier=1,Intent=Supplier