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 subcategories 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 by 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.