This is a CHARMM AtomType object (see Section 6.23), which tracks per-type rather than per-atom properties such as the element symbol, radius and atomic mass. If the model has not yet had CHARMM atom types assigned, it is None.
The atom type can also be changed by assigning a CHARMM atom name (as a string, such as 'CT1'), or None, to it. This can be useful when using BLK residues, which otherwise use the same atom type (`undf') for every atom.
(Only available for models, not template structures.)