An APP file is an application bundle that runs under macOS, an operating system (OS) developed by Apple and designed to run on Macintosh computers. It stores multiple directories, such as Contents, Resources, and Frameworks, which contain various files that make up the application. APP files are executable files similar to .EXE files in Windows and run when double-clicked.

Because APP files are macOS packages, you can right-click (or control-click) an application and select "Show Package Contents" to view the files contained in the package. These include resources used by the application, executables, property list (.PLIST) files, frameworks, plugins, icons, and other files. In Windows and Linux systems, it may be possible to view the contents of a macOS application as if it were a folder. A decompression program like WinRAR may also be able to extract files from a macOS application on other operating systems. NOTE: Files within an application package should not be edited or moved, since changes may prevent the program from running correctly.

Software program written for the Symbian OS, an operating system for mobile phones and smartphones; comprises a Symbian application together with a .RSC resource file; can be compiled using a Symbian OS software development kit (SDK).

The Symbian OS is used by phones from Nokia, Sony Ericsson, Samsung, and FOMA.

An APP file contains encrypted data dumped from a Nintendo Wii U video game. It stores a section of the game data, along with other APP files, that when decrypted and pieced together, can be opened and played on a computer using a Wii U emulator.

APP files are used by gamers who enjoy playing and altering Wii U games on their computers. Since APP files contain pieces of a Wii U game to reduce file sizes, all the APP files need to be present in order to play the Wii U game. Also, Wii U APP files are encrypted and must be decrypted before they can be opened and played. A Title Metadata .TMD file, which stores metadata and the title key for the APP game, is required in order to decrypt the APP file. APP files are named according to the number of files to which the game data was split. The files start at 0 (00000000.app), then increase from there to 00000001.app, 00000002.app, etc.