Design / Ordering-Type (RFC 3648)
When a collection is created, the client MAY request that it be ordered and specify the semantics of the ordering by using the new Ordering-Type header with a MKCOL request. For collections that are ordered, the client SHOULD identify the semantics of the ordering with a URI in the Ordering-Type header, although the client MAY simply set the header value to DAV:custom to indicate that the collection is ordered but the semantics of the ordering are not being advertised.