The value of 'xcmCommsOptionIndex' corresponding to the last
active configuration option (in 'xcmCommsOptionTable'), or zero
if this communications engine does NOT require such information.
Usage: This 'chain' represents currently active configuration
options of this communications engine.
Usage: The last entry index explicitly bounds the valid range
of 'xcmCommsOptionIndex'. This is the value of the numerically
highest entry index, NOT necessarily the 'logically last' entry
index in the structured tree in 'xcmCommsOptionTable'.