The USN_VECTOR structure defines a concrete type for the cookie (section 1.3.2) used to pass state between calls to IDL_DRSGetNCChanges.

 typedef struct {
   USN usnHighObjUpdate;
   USN usnReserved;
   USN usnHighPropUpdate;

usnHighObjUpdate: A USN.

usnReserved: A USN.

usnHighPropUpdate: A USN.

The USN_VECTOR type, as shown, is used in the DRS IDL. However, only the size of USN_VECTOR (24 bytes) and the representation of its null value (24 zero bytes) are standardized for interoperability.