A01 file extension is associated with file archives created using ARJ. ARJ tool was developed when MS-DOS was the most popular operating system on the market, when data was stored on floppy disks (limited to 1,44 MB of storage capacity).

A01 files is the third part of multi-volume archive. The first volume used ARJ extension, and the second A00 extension. All subsequent volumes are numbered according to this naming scheme (A02, A03, etc.). Splitting archive into smaller parts allows to store data on multiple low-space storage devices. Split archives can only be unpacked when all of its parts are present.