Manual pages can contain references (using .so or .mso) to files that
can be included in the parent manual page. Those files can be at an
absolute path or a path relative to the base manual page path for the
current language or to the manual page macro path. Since these
directives could include any file from the filesystem including from
other packages, adequate should check that the files exist on the
filesystem to ensure there will be no content missing from manuals.

https://www.gnu.org/software/groff/manual/html_node/I_002fO.html