The value of 'xcmCommsOptionIndex' corresponding to a previous
associated conceptual row in the 'xcmCommsOptionTable', or zero
if this is the first associated conceptual row in a given set.
Usage: This object provides common 'backward' linkage for BOTH
the 'xcmCommsOptionNextIndex' and 'xcmCommsOptionFamilyIndex'
linkage objects. A previous conceptual row MAY 'forward'
reference this conceptual row via either '...NextIndex' or
'...FamilyIndex' - ie, a given conceptual row MAY 'forward'
reference EXACTLY zero, one, or two 'later' conceptual rows.