An A01 file is usually the second part of a split archive where a larger file was broken into numbered chunks, and the easiest way to identify it is by checking for sibling files with the same base name—if you see a .ARJ plus .A00, .A01, .A02, etc., it’s almost certainly an ARJ multi-volume set where .ARJ is the main index and the numbered files store the data, meaning extraction should start from the .ARJ, not A01; if no .ARJ exists but .A00 and higher numbers are present, it still points to a split set where .A00 is the first volume, and tools like 7-Zip or WinRAR can confirm by opening the starter file, with failures often caused by missing parts or gaps in the sequence, which indicates A01 is just a fragment, not a standalone file.

A "split" or "multi-volume" archive is a single archive broken into multiple parts so it’s easier to store, upload, or send under size limits, producing files like `backup.a00`, `backup.a01`, `backup.a02` that each hold a continuous slice of data; in that arrangement, A01 is typically volume two and can’t open by itself because key structure and file-list data live in the first volume or main index (such as an `.ARJ` plus `.a00/.a01` files), and extraction software must start at the first chunk, pulling later volumes in order—with missing or corrupted parts causing errors like "unexpected end of archive" since the full sequence can’t be rebuilt.

You often see an A01 because older archivers and splitters favored a numbering scheme where the extension reflects the volume order instead of a standalone format, making A00 the first slice, A01 the second, and so forth, allowing easy reconstruction; this is common in ARJ multi-volume archives where .ARJ holds the index and A00/A01 contain data, and in various backup workflows using "Axx," so A01 naturally appears whenever a second volume exists, especially when the true starting file is overlooked or missing.

To open or extract an A01 set correctly, note that A01 typically lacks the archive’s header, so you need the volume that starts the sequence; confirm that each file is present and follows the expected naming (`backup.a00`, `backup.a01`, `backup.a02`), then start extraction from the `.ARJ` file if one exists, or else from `.A00`, letting your archive tool read the remaining volumes in order, and if you hit "unexpected end of data" or CRC issues, it usually means a missing segment, a numbering gap, or corruption.

To confirm what your A01 belongs to rapidly, open the directory and sort alphabetically so similar files cluster, then check whether the same base name appears on a .ARJ plus .A00/.A01/.A02, which strongly signals an ARJ set where .ARJ is the proper opener; if no .ARJ is present but .A00 is, treat .A00 as the starter and right-click → 7-Zip/WinRAR → Open archive to verify, and also look for uninterrupted numbering and comparable file sizes because missing pieces often cause extraction errors.