Its not, but such is the tangled history of Unix's first file system. FFS technically refers to a lower-level 'backend' part of UFS. UFS was structured to allow an alternate log-structured LFS backend too, but I haven't seen it for years and few systems supported it. So if you had both available you'd differentiate between them as FFS or LFS as they were both technically UFS. AmigaOS had a FFS as well but I don't think that is any relation, and there is no one UFS - just each operating system's own variant.