The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
The menu item has a line that specifies an unknown section or uses a section that is intended only as a menu root, not as a section that applications should use directly. Check the spelling of the section and check the section against the list in the menu policy. (The menu sections changed as of June of 2007.)
Refer to The Debian Menu sub-policy section 2.1 for details.