A relatively generic description of the current position of this protocol entity (ie, this layer) in this protocol stack. Usage: A conceptual row in 'xcmCommsStackTable', which occupies a 'bottom' position in a protocol stack AND has a corresponding row in the 'xcmCommsStackXrefTable', SHOULD have valid references in 'xcmCommsStackXrefIfIndex' (to IETF MIB-II) and 'xcmCommsStackXrefHrCommDevIndex' (to IETF Host Resources MIB). Usage: A conceptual row in 'xcmCommsStackTable' which occupies a 'lowerMux' and/or an 'upperMux' position in a protocol stack SHALL have one (or two) valid corresponding conceptual rows in the 'xcmCommsMuxTable', as the conventionally used 'xcmCommsStack[Lower|Upper]StackIndex' objects take on zero values for multiplexors (thus breaking the graph of the stack layers, without the use of the 'xcmCommsMuxTable').