A read-and-write property identifying to which Namespace this Identifier refers. This index must be between 0 (zero) and the number of Namespaces minus 1 (one) in a server. When defining a Field in a Query, the index refers to the OPC UA Namespaces Collection defined in a Query object, which at some point may be different from the Collection of Namespaces of a server. This value cannot be greater than 65535, because the OPC UA standard defines that this is a 16-bit value.