FHS 3.0 was released June 3, 2015 on /var/lib
APT HOWTO (Obsolete Documentation) How to deal with errors
/srv/ - not part of FHS, site-specific data, served by the system /usr/ - secondary hierarchy for shareable, read-only data, formerly from UNIX source repository, now from UNIX system resources, -- files that are not required to boot or rescue the system. the same debian wiki page](https://wiki.debian.org/FilesystemHierarchyStandard)
Various notes on /usr unification / February 28, 2012 "/usr unification" (or simply "usrmove") is the idea of moving the contents of /bin, /lib, and related root-level directories into their equivalents under /usr.
The case for the /usr merge / January 26, 2012]https://lwn.net/Articles/477467) Lennart Poettering has announced the posting of a summary of the motivations for merging several root-level directories into /usr.
- Oracle Solaris has implemented the /usr merge in parts 15 years ago, and completed it in Solaris 11. Fedora is following suit here, it is not the pioneer
- The /usr merge makes sharing the vendor-supplied OS resources between a host and networked clients as well as a host and local light-weight containers easier and atomic. Snapshotting the OS becomes a viable option. The /usr merge also allows making the entire vendor-supplied OS resources read-only for increased security and robustness.
- /usr - installed system; shareable; possibly read-only
- /etc - config data; non-shareable
- /var - persistent data; non-shareable;
- /run - volatile data; non-shareable; mandatory tmpfs filesystem
5 March 2019 / Debian Sticking With Merged /usr Plan For years Debian developers have been planning for a merged /usr concept where the /{bin,sbin,lib}/ directories becoming symbolic links to /usr/{bin,sbin,lib}/. With the upcoming Debian 10 Buster is the initial step of their plan after it was removed from from Debian Stretch RC1(15 January 2017) TC decision on "Merged /usr" - #914897
https://ostree.readthedocs.io/en/latest/contributing-tutorial
https://linuxhint.com/fedora-_atomic_workstation_review (Fedora 28 - May 2018)
see also