In their paper, GOR mention that if you know something about the secondary structure content of the protein you are analyzing, you can do better in prediction. 'idc' is an index into a set of arrays, dharr[] and dsarr[], which provide 'decision constants' (dch, dcs), which are offsets that are applied to the weights for the helix and sheet (extend) terms. So, idc=0 says don't use the decision constant offsets, and idc=1 to 6 indicates that various combinations of dch,dcs offsets should be used