The Heating Degree Days (HDD) and Cooling Degree Hours (CDH) data in REM comes from two places - first we get a reported value from a source such as ASHRAE or the web and that is the value you see when you pick a site in REM.  These values are used for envelope insulation levels in older energy codes (prior to 2003).   More current energy codes use climate zones to determine building requirements.  There is another HDD and CDH value in the REM climate file which are calculated from the TMY2 weather data that is used in the simulation for some types of heating analysis.  

REM does not calculate the heating and cooling loads based on the HDD and CDH, but does use them for other parameters like heating consumption part load values.