Unix permissions is not a capability system though. Capabilities are more like "here is a file descriptor pointing to a directory, you are not capable of referring to anything outside it". So closer to chroot, except you can have several such directory references at the same time.

You can always narrow down a capability (get a new capability pointing to a subdirectory or file, or remove the writing capability so it is read only) but never make it more broad.

In a system designed for this it will be used for everything, not just file system. You might have capabilities related to network connections, or IPC to other processes, etc. The latter is especially attractive in microkernel based OSes. (Speaking of which, Redox OS seems to be experimenting with this, just saw an article today about that.)

See also https://en.wikipedia.org/wiki/Capability-based_security