This field should be assigned as a unique member number
within a redundancy group. The value 0 always indicates a
secondary member. Primary members should have numbers
which are higher than secondary members.
Note: This definition of member values, including the use
of the value 0 for the secondary member allows compatibility
with existing 1:n SONET APS channel numbering. Yet the
numbering definition has also been expanded to allow support
for the most general m:n redundancy architectures.