The value of 'xcmCommsStackIndex' corresponding to a next higher associated conceptual row in the 'xcmCommsStackTable', or zero if this stack layer's position is either 'upperMux' or sometimes 'top' (see usage note below). Usage: Only for protocol entities whose current purpose is either 'layerInterWorkingUnit' (ie, relays or gateways) or 'systemInterWorkingUnit' (ie, application gateways), it is permitted that the stack layer whose current position is 'top' have an upper layer index pointing to the peer entity (also in a 'top' position) which comprises the 'other half' of a relay or gateway between two different address domains (ie, an active protocol conversion relay) - that is the graphed protocol stack is an inverted 'U'.