The tag is present in Lintian version 2.104.328. 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.
Please refer to Debian doc-base Manual section 22.214.171.124 and Debian doc-base Manual section 126.96.36.199 for details.