001 suffix is used by multipart archives for the second part of split archive. 001 extension is used by various file archive managers.

As a rule, each part of multipart archive, beside the first part, which uses suffix native to given archiver, for example, ZIP, RAR, or 7z, is given a suffix based on the 001, 002, 003, etc. sequence. In order to extract such archive, all its parts must be present in the same folder. Split archives are commonly used when for transporting large files using multiple smaller storage devices.