The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
The doc-base control file contains field which is either unknown or not valid for the section where was found. Possible reasons for this error are: a typo in field name, missing empty line between control file sections, or an extra empty line separating sections.
Refer to Debian doc-base Manual section 126.96.36.199 and Debian doc-base Manual section 188.8.131.52 for details.