Found the answer, (Source: Barry's Book 2nd edition, pg 125).
"Pressing the REL soft key allows toggling on and off Relative speed steps and causes the thumb wheel to act like the red knob on the CAB-1 remote. This is useful for operation of the TMCC Crane car. When Relative speed steps is selected, the REL soft key will remain highlighted and REL will appear in the DCS Remote's LCD screen instead of an actual TMCC speed step number. Pressing REL a second time will restore the default Absolute speed steps and the speed steps number will reappear."
Comment: Thank you Barry for the work you did for this book, I would be lost without it. Recommend it for all DCS users.