A field within a structure can itself be a structured item composed of other fields, other structures, or both. You can declare a substructure in two ways:
One or more field names must be defined in the STRUCTURE statement for the substructure, because all fields in a structure must be named. In this case, the substructure is being used as a field within a structure or union.
Field names within the same declaration nesting level must be unique, but an inner structure declaration can include field names used in an outer structure declaration without conflict.
See the example in the Section B.12.1 for a sample structure declaration containing both a nested structure declaration (TIME) and an included structure (DATE).