systemd-remount-fs.service is an early boot service that applies mount options listed in fstab(5) to the root file system, the /usr file system, and the kernel API file systems. This is required so that the mount options of these file systems --- which are pre-mounted by the kernel, the initial RAM disk, container environments or system manager code --- are updated to those listed in /etc/fstab. This service ignores normal file systems and only changes the root file system (i.e. /), /usr and the virtual kernel API file systems such as /proc, /sys or /dev. This service executes no operation if /etc/fstab does not exist or lists no entries for the mentioned file systems.
For a longer discussion of kernel API file systems see m[blue]API File Systemsm.
API File Systems