Has anyone been successful in being able to add fields into this Customize Sublist list? For example this is the Pay Bills screen and when you click customize you can use a lot of the fields on the Bill record as a filter or column. Also you can add a few Vendor record fields as there is a vendor join in the list I’d like to create a custom record joined to the vendor bill and use a field from that custom record as a filter in the pay bills screen. Any ideas on how to do that?

You cannot customize what fields appear there - your best bet sounds like a custom sourced field on the bill from the custom record so that you can just select the custom bill field.