The default subvolume of a Btrfs
volume is special. When you mount, you normally have to name a
subvolume to mount. When you don't name a subvolume, the default subvolume will be used. The default subvolume can be changed to any subvolume. It is advisable to set that subvolume as default, that is used for mounting linux
"/" this is often the subvolume with the name
"@"