Data type: char
Must appear in a looped list containing '_atom_site_label'
Component 0 is normally a code which matches identically with one of the _atom_type_symbol codes. If this is the case then the rules governing the _atom_type_symbol code apply. If, however, the data item _atom_site_type_symbol is also specified in the atom site list, component 0 need not match this symbol or adhere to any of the _atom_type_symbol rules. Component 1 is referred to as the "atom number". When component 0 is the atom type code, it is used to number the sites with the same atom type. This component code must start with at least one digit which is not followed by a + or - sign (to distinguish it from the component 0 rules). Components 2 to 6 contain the identifier, residue, sequence, asymmetry identifier and alternate codes, respectively. These codes may be composed of any characters except an underline.