O ponto de montagem legado geralmente é usado quando os sistemas de arquivos são montados usando fstab
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Da página de manual do FreeBSD:
If needed, ZFS file systems can also be managed with traditional tools (mount(8), umount(8), fstab(5)). If a file system's mount point is set to legacy, ZFS makes no attempt to manage the file system, and the administrator is responsible for mounting and unmounting the file system.