When a file is split, The Vault selects a name for each of its parts. The root of this name (that is, the part before the `.') is the same as the root of the file being split. The extender has a unique id letter chosen from A to Z followed by a 2-digit part number. Thus, the first part of the file FOO.BAR may be called FOO.A01, in which case the second part would be called FOO.A02, and so on. If another file or part such as FOO.A99 already exists that might conflict with the generated part names, The Vault will use B instead of A as the unique id letter. If that doesn't work, it tries C, and so forth. It keeps doing that until it find a letter that works. If it can't use any letter, it gives an error. Note that if two files such as FOO.PRG and FOO.RSC both exist and both need to be split, the parts of one would be named FOO.A01, ... and the parts of the other would be named FOO.B01, ... .
Each part of a split file contains a one-line, 22-byte header describing what file it is a part of, what its part number is, and whether it is the last part. This information is used for putting a split file together when restoring it.