Notes

^1 While the original tar format uses the ASCII character encoding, current implementations use the UTF-8 (Unicode) encoding, which is backwards compatible with ASCII.
^2 Supports the external Parchive program (par2).
^3 From 3.20 release RAR can store modification, creation and last access time with the precision up to 0.0000001 second (= 0.1 µs). WinRAR download and support: Knowledge Base
^4 The PAQ family (with its lighter weight derivative LPAQ) went through many revisions, each revision suggested its own extension. For example: ".paq9a".
^5 WIM can store the ciphertext of encrypted files on an NTFS volume, but such files can only by decrypted if an administrator extracts the file to an NTFS volume, and the decryption key is available (typically from the file's original owner on the same Windows installation). Microsoft has also distributed some download versions of the Windows operating system as encrypted WIM files, but via an external encryption process and not a feature of WIM.

• Purpose: Archive formats are used for backups, mobility, and archiving. Many archive formats compress the data to consume less storage space and result in quicker transfer times as the same data is represented by fewer bytes. Another benefit is that files are combined into one archive file which has less overhead for managing or transferring. There are numerous compression algorithms available to losslessly compress archived data; some algorithms are designed to work better (smaller archive or faster compression) with particular data types. Archive formats are used by most operating systems to package software for easier distribution and installation than binary executables.
• Filename extension: The DOS and Windows operating systems required filenames to include an extension (of at least one, and typically 3 characters) to identify the file type. Such extensions must be unique for each type of file. Many operating systems identify a file's type from its contents without the need for an extension in its name. However, the use of three-character extensions has been embraced as a useful and efficient shorthand for identifying file types.
• Integrity check: Archive files are often stored on magnetic or other media subject to storage errors. Many archive formats contain extra error detection or correction information which can be used by the software used to read the archive files to detect and possibly correct errors.
• Recovery record: Many archive formats contain redundant data embedded in the files in order to detect data storage or transmission errors, and the software used to read the archive files contains logic to detect and correct errors.
• Encryption: Many archive formats include the capability to encrypt contents to prevent unauthorised access, using one of many available encryption methods.

Notes

^1 Not to be confused with the archiver JAR written by Robert K. Jung, which produces ".j" files.