The tag is present in Lintian version 2.109.3. That is the most recent version we know about.
The given source file is licensed under GFDL with invariant section or front-cover or back-cover text.
GFDL with invariant sections, front-cover or back-cover texts are not suitable for main.
If this file be multi-licensed, please override the tag.
Please refer to https://wiki.debian.org/qa.debian.org/gfdlinvariant and https://www.debian.org/vote/2006/vote_001 for details.