The FHS bases its layout specification on the idea that there are two independent categories of files: shareable vs. unshareable, and variable vs. static. Shareable data can be shared between hosts; unshareable data is specific to a given host (such as configuration files). Variable data can be modified; static data is not modified (except at system installation and maintenance).

The following grid summarizes the four possible combinations, with examples of directories that would fall into those categories. Again, this table is straight from the FHS specification:

    +---------+-----------------+-------------+
    |         | shareable       | unshareable |
    +---------+-----------------+-------------+
    |static   | /usr            | /etc        |
    |         | /opt            | /boot       |
    +---------+-----------------+-------------+
    |variable | /var/mail       | /var/run    |
    |         | /var/spool/news | /var/lock   |
    +---------+-----------------+-------------+