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The output of your command "$ man projects" is:

projects(5)                   File Formats Manual                   projects(5)

NAME
       projects - persistent project root definition

DESCRIPTION
       The  /etc/projects file provides a mapping between numeric project iden‐
       tifiers and those directories which are the roots  of  the  quota  tree.
       Its format is simply:

            # comments are hash-prefixed
            # ...
            10:/export/cage
            42:/var/log

       The  /etc/projects  file  is  optional, instead xfs_quota(8) can be used
       with the -p argument to specify a project root directly for each  opera‐
       tion.

SEE ALSO
       xfs_quota(8), xfsctl(3), projid(5).

                                                                    projects(5)