I have two suggestions here. See if any of it can be implemented for your use case or not. One option is to - have a custom content type for billing, and these type of contents should be added inside Bills folder. This way, just using the TYPE query for billing content type, you should be able to find all the Billing documents. Alternatively, if above option is not possible then, you can setup an inbound business rule on Bills folder and add new billing aspect to the documents. And then search for all the documents having that aspect. This way, you should be able to get the documents from Bills folder.