G.Bond port mode of operation (subtype).
The value of 'subscriber' indicates the port is designated as
'-R' subtype (all BCEs assigned to this port are of subtype
'-R').
The value of the 'office' indicates that the port is
designated as '-O' subtype (all BCEs assigned to this port are
of subtype '-O').
The value of 'unknown' indicates that the port has no assigned
BCEs yet or that the assigned BCEs are not of the same side
(subTypeBCEMismatch).
This object maps to the TR-159 attribute aGroupEnd.