OS X Application and System Extension File
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What is an APPEX file?
An APPEX file is an extension bundle created to add functionality to an application built in Xcode, a software development IDE used for developing iOS and OS X apps. It contains an extension which consists of code used to perform a task such as posting to a social channel from a web browser, replacing the iOS default keyboard with a custom keyboard, or providing access to and managing a repository of files. APPEX files are packaged inside their containing .APP files.
APPEX (short for app extension) files allow developers to extend the custom functionality and content beyond their app. App extensions also make the content from a developer's app available to users while they are utilizing the system or other apps. Since Apple applies limitations to the memory usage of apps, extensions are utilized to launch separately from the app to provide functionality then quit in order to save on memory use. App extensions are opened when a user selects it in a host app, which then issues a request to the extension, which receives it, assists the user in performing the task, then promptly closes after completing or cancelling the request.