              Data items in the ENTITY category record details (such as\
               chemical composition, name, and source) about the molecular\
               entities that are present in the crystallographic structure.\
\
               Items in the various ENTITY sub-categories provide a full\
               chemical description of these molecular entities.\
\
               Entities are of three types:  polymer, non-polymer and water.\
               Note that the water category includes only water;  ordered\
               solvent such as sulfate ion or acetone would be described as\
               individual non-polymer entities.\
\
               The ENTITY category is specific to macromolecular CIF\
               applications, and replaces the function of the CHEMICAL category\
               in the CIF core.\
\
               It is important to remember that the ENTITY data are not the\
               result of the crystallographic experiment;  those results are\
               represented in the ATOM_SITE data items. ENTITY data items\
               describe the chemistry of the molecules under investigation,\
               and can most usefully be thought of as the ideal groups to which\
               the structure is restrained or constrained during refinement.\
\
               It is also important to remember that entities do not correspond\
               directly to the enumeration of the contents of the asymmetric\
               unit. Entities are described only once, even in those structures\
               that contain multiple observations of an entity. The\
               STRUCT_ASYM data items, which reference the entity list,\
               describe and label the contents of the asymmetric unit.