Servers MUST support the use of non-standard component, property, or parameter names in the CALDAV:calendar-data XML element in calendaring REPORT requests to allow clients to request that non- standard components, properties, and parameters be returned in the calendar data provided in the response.
Servers MAY support the use of non-standard component, property, or parameter names in the CALDAV:comp-filter, CALDAV:prop-filter, and CALDAV:param-filter XML elements specified in the CALDAV:filter XML element of calendaring REPORT requests.
Servers MUST fail with the CALDAV:supported-filter precondition if a calendaring REPORT request uses a CALDAV:comp-filter, CALDAV:prop- filter, or CALDAV:param-filter XML element that makes reference to a non-standard component, property, or parameter name on which the server does not support queries.