Spanned records are preceded by control fields, as shown below.

Each block is preceded by a 4-byte block descriptor field ('BDF' in the image). There is only one block descriptor field at the beginning of each physical block.
Each segment of a record in a block, even if the segment is the entire record, is preceded by a 4-byte segment descriptor field ('SDF' in the image). 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 of your COBOL program, and the fields are not available for you to use in your program.
related tasks
Requesting spanned format
related references
Layout of format-F records
Layout of format-V records
Layout of format-U records