The length of a subfield may be specified using absolute (positional) or
length notation, or its length may be implied.
Absolute
Specify a value in both the From-Position (positions 26 through 32)
and the To-Position/Length (positions 33 through 39) entries on the definition
specification.
Length
Specify a value in the To-Position/Length (positions 33 through 39)
entry. The From-Position entry is blank.
Implied Length
If a subfield appears in the first parameter of one or more OVERLAY keywords, the subfield can be defined without specifying any type
or length information. In this case, the type is character and the length
is determined by the overlaid subfields.
In addition, some data types,
such as Pointers, Dates, Times and Timestamps have a fixed length. For these
types, the length is implied, although it can be specified.
When using length notation, the subfield is positioned such that its starting
position is greater than the maximum To-Position of all previously defined
subfields. For examples of each notation, see Data Structure Examples.