The identification-sequence field can contain identification characters or sequence numbers or both. If the ISEQ instruction has been specified to check this field, the assembler verifies whether the source statements are in the correct sequence.

The columns that are checked by the ISEQ function are not restricted to columns 73 through 80, or by the boundaries determined by any ICTL instruction. The columns that are specified in the ISEQ instruction can be anywhere on the input statement, including columns that are occupied by the statement field.