The Sig-Len header defines the length of the "personal signature" often found at the end of plain-text USENET articles, not to be confused with any digital authentication signature.
The personal signature is a piece of text typically appended to the end of all articles by a given poster by their authoring software. This text is not part of the body of the article, and usually does not vary from article to article, but may. It often contains personal information about the author in free format text.
In the past, it has been delimited from the body by a line consisting solely of two dashes and a space:
Sig-Delim = "-- "
Many other ad-hoc delimeter forms have also appeared.
All posting agents which add a personal signature SHOULD, and after Signature-Formalize-Date MUST, add a "Sig-Len" header to the article header. This header contains one or two ascii integer values, delimited by FWS.
The first is the number of total lines of personal signature, including any lines of delimiter. If the last line of the signature does not, for some reason, have a terminating newline, it shall still be counted.
The second integer is the size of the delimiter region of the signature. The delimiter may be altered or specially formatted when displaying an article. Newsreading software MAY decline to display the personal signature at all. The default is 0, unless the first line is the Sig-Delim, in which case it is 1.
Articles with more than one signature (eg. that of a poster and a moderator) should provide the length of the combined signature and all delimiters.
Posting agents SHOULD, prior to Signature-Formalize-Date, include the one-line Sig-Delim specified above in the personal signature. The Sig-Delim MUST NOT appear in the body of the article and SHOULD not appear in the personal signature.
Search Engine tools SHOULD NOT index personal signature data along with article text, though they MAY make it available for search upon request.
Followup-Agents SHOULD NOT, unless requested, when preparing included text from a parent article for inclusion in a followup, insert text from the personal signature. If the Sig-Len header is not present, they MAY do their best to guess the division between text and personal signature, using the first instance of the Sig-Delim.
It is expected in the future that MIME "multipart/mixed" articles may place the personal signature in an appropriately tagged part of the multipart, in which case this header MUST NOT be used. It should only be applied to articles of the MIME "text" type, including the default "text/plain."
Note: This header is intended to be a temporary measure until such time as a more formal method for defining the structure of the text of an article becomes commonplace. Parsing of delimiters within articles is not reliable.
There is debate about whether the location of the personal signature should be formalized through the use of yet another header. Use of just the delimiter is not 100% reliable, but involves no new header.