Page 12 if heat pump
http://customer.honeywell.com/techlit/pdf/PackedLit/69-1900EF.pdf
what color wire was on or is it the new tstat
I check with my supply house and the answer that they gave is that you will not need that wire it was use for an outdoor thermsister that was series with the heat anticipator that during colder weather it held it on for longer run periods
Thank You good luck
HVAC Technician
35 years experience, HVAC