When the custom_field_category parameter is used with the GET Constituent List endpoint, the records returned are constituent who have that specific attribute on their record in the database.

For example, custom_field_category=Interests returns constituents with an attribute category of "Interests".

At this time, we are not able to filter on custom field values. For example, we cannot pull constituents who have the attribute category of "Interests" and the attribute value of "Soccer".