This query allows searching by any property of the items in the list. Top-level properties such as display_name, external_reference and category are supported. It also supports the property names inside the properties array (provided the name doesn't match a top-level property). These property names will differ from list to list, and so cannot be defined in advance. All matches are partial, e.g. a query parameter of privacyPolicyRef=nov will match items with entries in the property array under key privacyPolicyRef having values of 'nov2017', 'nov17', or 'november_2017'. Hence, passing in an empty string will match items with any value for a given key.