The specification defines either a data structure subfield or a parameter
within a prototype or procedure interface definition.
C
The specification defines a constant. Position 25 must be blank.
DS
The specification defines a data structure.
PR
The specification defines a prototype and the return value, if any.
PI
The specification defines a procedure interface, and the return value
if any.
S
The specification defines a standalone field, array or table. Position
25 must be blank.
Definitions of data structures, prototypes, and procedure interfaces end
with the first definition specification with non-blanks in positions 24-25,
or with the first specification that is not a definition specification.
For a list of valid keywords, grouped according to type of definition,
please refer to Table 52.