fs_type is considered a type of file system which is only used in implementation of the OS.
This is primarily used for hiding mounted volumes that are intended as APIs for programs to read, and system administrators at a shell; rather than something that should, for example, appear in a GUI. For example, the Linux `/proc` filesystem.
The list of file system types considered ‘system’ ones may change over time.
a file system type, e.g. `procfs` or `tmpfs`