Package kernel-modules-subfs-std-def: Information

Subfs is the kernel portion of the submount removable media handling system.
For submount to function, it needs both the subfs kernel module and the
submount package to be installed.

Submount is a system for automatically mounting and unmounting
removable media drives like CD-ROMs, flash and floppy disk drives. Once
installed, it allows removable media drives to be accessed as if they
were permanently mounted.