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FHS-3.0 describes it as:
Shareable, readonly data. That means that
/usrshould be shareable between various FHS-compliant hosts and must not be written to.
I am a bit confused by what this means. Does this means that the binaries or whatever other files inside should be copy-pasteable onto another machine, and have them function perfectly fine?
I think what it means is only that distros shouldn't assume that an installation has sole ownership of /usr, not that everything in /usr is expected to work with all FHS-compliant systems. I think I have heard of /usr being served over the network (via NFS for example) for a bunch of systems running the same distro. Since /usr is where the bulk of all installed files reside, this makes for a lot of space savings. Also, I think it's not unusual to have /usr a separate filesystem in any case, mounted read-only for additional security, so the "must not be written to" part helps with that as well.
/etc can't be shared in this manner - some files, like /etc/hostname are necessarily different for each host (though most files in /etc can be so shared, I think). Nor can /var - it wouldn't make sense to have two services on different systems logging to the same file, for example.
