The “2” in the *.devhelp2 file extension is because it is the
second version of the file format. The first version of the format, with
the *.devhelp file extension, is deprecated and its support in
Devhelp may be removed in the future. On application startup,
when Devhelp scans the filesystem to find books, it emits a
warning message in the terminal for each book that uses a deprecated
format.
These contain links to the HTML files to reach the corresponding pages and
symbols.