An A01 file is most often volume 2 in a split archive chain, and the fastest way to figure out what it belongs to is by spotting files with matching names—seeing .ARJ together with .A00, .A01, .A02 strongly signals an ARJ set where the .ARJ is the controller/index and the numbered volumes store the payload, so extraction begins with the .ARJ; if no .ARJ is present but .A00 and .A01 are, it still suggests a split set where .A00 must be opened first, and a quick test using 7-Zip or WinRAR helps confirm, with errors usually caused by missing segments or incomplete sequences, showing that A01 is just one piece of a larger whole.

A "split" or "multi-volume" archive refers to an archive divided into several volumes to meet size restrictions, generating files like `backup.a00`, `backup.a01`, `backup.a02` where each volume carries a portion of the data; A01 in that context is simply the second volume and won’t open alone because the initial structure and index reside in the first chunk or a main file like `.ARJ`, so extraction tools begin with `.ARJ` or `.A00` and fetch volumes in order, failing with errors such as "unexpected end of archive" if any segment is missing or damaged.

You often see an A01 as older file-splitting systems assign filenames based on part order rather than distinct formats, producing A00 as volume one, A01 as volume two, and onward, simplifying multi-part reconstruction; ARJ workflows frequently use this model with .ARJ as an index file and the Axx files carrying the data, and the same logic appears in backup splitters, so A01 is common whenever two or more volumes were created, especially if the initial .ARJ or .A00 isn’t noticed or shared.

To open or extract an A01 set correctly, keep in mind A01 doesn’t contain the archive’s opening structure, so begin with the correct first piece; ensure all parts are in one folder, properly named (`backup.a00`, `backup.a01`, `backup.a02`), choose the `.ARJ` as the entry file when present (otherwise `.A00`), and open it via 7-Zip/WinRAR so the tool can assemble the rest automatically, with common extraction failures pointing to absent volumes, corrupted copies, or incompatible extraction software.

To confirm what your A01 belongs to efficiently, open the folder and sort by Name so matching parts align, then check if a .ARJ sits with .A00/.A01/.A02, marking it as an ARJ multi-volume archive where .ARJ is the starter; otherwise, if .A00 appears without .ARJ, it’s likely a simple split set beginning with .A00—right-click it and try 7-Zip/WinRAR → Open archive to confirm, and ensure the numbered sequence is complete because gaps or mismatched sizes usually cause extraction to fail.