The file system used in DOS, OS/2 and many versions of Windows. The name comes from the "file allocation table" that is used to to keep track of the data on the disk. See FAT and FAT32 .

Should that be the case, and should you now wish to convert the file system to FAT to also permit using the device with a Windows PC, open the macOS Disk Utility by opening Finder, selecting ...

MS-DOS File System is the older FAT filesystem used with MS-DOS and Windows. Note that you can't have files of 4GB or bigger on a FAT volume. UNIX File System (UFS) is exactly what the name suggests.