Layout of format-S records
Spanned records are preceded by control fields, as explained below.
Each block is preceded by a 4-byte block descriptor field ('BDF' in the image above). There is only one block descriptor field at the beginning of each physical block.
Each segment of a record in a block is preceded by a 4-byte segment descriptor field ('SDF' in the image) even if the segment is the entire record. There is one segment descriptor field for each record segment in the block. The segment descriptor field also indicates whether the segment is the first, the last, or an intermediate segment.
You do not describe these fields
in the DATA DIVISION, and the fields are not available
for you to use in your COBOL program.