The PDB format reserves four characters in columns
73-76 to specify a segment ID, e.g. to distinguish the light and heavy chains in antibodies. In old PDB files,
the segment ID is often the same as the PDB ID.
Examples:
ColorMol Segment HEAV, Green
Color all molecules belonging to segment 'HEAV' green
ListRestAtom Segment " A",Distance
List all NMR distance restraints involving atoms in segment A