It means that the contents of those directories are not to be exported across a network, because then problems could arise at the local network node that would be importing the share.
To give you an example, files with regard to local authentication ─ e.g. /etc/passwd and friends ─ should not be shared across the network. /usr and /opt are shareable because they contain only software, documentation, et al.
In and of itself, it has nothing to do with partitioning, although there’s a caveat in Manjaro and Arch if you want to split off /usr from the root filesystem. You’ll need to modify /etc/mkinitcpio.conf for that and manually (re)build the initramfs.