systemd-remount-fs.service is an early boot service that applies mount options listed in fstab(5), or gathered from the partition table (when systemd-gpt-auto-generator(8) is active) 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 configured in /etc/fstab and the other sources. 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 no configuration is found (/etc/fstab does not exist or lists no entries for the mentioned file systems, or the partition table does not contain relevant entries).
For a longer discussion of kernel API file systems see m[blue]API File Systemsm.
Note: systemd-remount-fs.service is usually pulled in by systemd-fstab-generator(8), hence it is also affected by the kernel command line option fstab=, which may be used to disable the generator. It may also pulled in by systemd-gpt-auto-generator(8), which is affected by systemd.gpt_auto and other options.