1: planar atom, e.g. C and N in a peptide bond
5: planar atom in a 5-membered ring
6: planar atom in a 6-membered ring
11: planar atom at junction of 5- and 6-membered rings
12: planar atom at junction of two 6-membered rings
0: anything else
These identifications are used to determine which out-of-plane torsions to define.