9P is a filesystem protocol (like NFS or CIFS) rather than a local file system (like btrfs or NTFS). It's used to share files between the host and the guest. That way there's no need to copy files between the host and the guest.
replyI might be wrong since it has been a long time since i used WSL, but i think that WSL2 uses 9P for mounting between the VM and the host.
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