Under /usr you'll find a secondary hierarchy that looks a lot like the root filesystem. It isn't critical for /usr to exist when the machine powers up, so it can be shared on a network ("shareable"), or mounted from a CD-ROM ("static"). Most Linux setups don't make use of sharing /usr, but it's valuable to understand the usefulness of distinguishing between the primary hierarchy at the root directory and the secondary hierarchy at /usr.

This is all we'll say about the Filesystem Hierarchy Standard. The document itself is quite readable, so you should go take a look at it. We promise you'll understand a lot more about the Linux filesystem if you read it.