The tag is present in Lintian version 2.104.318. That is the most recent version we know about.
The source tarball contains a prebuilt Microsoft precompiled help file (CHM file). These are often included by mistake when developers generate a tarball without cleaning the source directory first.
CHM files are mainly produced by proprietary, Windows-specific software. They are also mainly consumed by the Microsoft HTML Help Workshop.
Whilst there is free software to read and write them, any examples existing in source packages are likely to be created by the proprietary Microsoft software and are probably missing the source HTML and associated files.
If there is no sign this was intended, consider reporting it as an upstream bug.