disktype detects selected archive formats, because they can occur on Linux installation disks, but also on backup tapes.
The tar (tape archive) format as originally created for backup to magnetic tape. Today, it is the Unix archive format and is widely used for software distribution as well as backup. It conserves all Unix file system specialties, including permissions and hard links. tar by itself does not include compression ans is not exactly space-efficient. Therefore it is usually combined with a file compressor like gzip, compress, or bzip2.
Several variations of the format exist. disktype identifies the base format using the checksum of the first header, then looks for signatures to identify the GNU and POSIX flavors. The code to do this was contributed by Doug Merritt.
cpio is another Unix archive format, originally created for backup purposes. Some variations use binary headers, while others encode numbers as ASCII. disktype should recognize most of these variations.
The dump and restore commands are used to back up a file system at a very low level, but short of a sector-by-sector copy. The data is very file system specific and in general can only be used by the same operating system and the same file system driver. disktype tries to identify some formats by their magic number.