A sequence number, normally consisting of six digits, can be placed in the sequence area and can be used to label a source line. This sequence number is usually in ascending numeric order on each successive source line.
This sequence number is the one used by the BASIS mechanism for line editing, in which case it must be both numeric and in ascending order through the program (see the topic The BASIS Mechanism).
The ascending order of sequence numbers can be optionally verified by your COBOL system by using the SEQCHK Compiler directive.
There is no requirement for the content of this area to be numeric, or even unique.
If the first character position of the sequence number field contains an asterisk, or any non-printing control character (less than the character SPACE in the ASCII collating sequence), then the line is treated as comment, and is not output to the listing file or device. This facility allows an output listing file to be used as a source file to a subsequent compile. This support is sensitive to the MFCOMMENT Compiler directive.