To offer another take at a summary: Apple has offered App Extensions for several years, in both UI-serving and non-UI contexts. App extensions are secondary se

To offer another take at a summary: Apple has offered App Extensions for several years, in both UI-serving and non-UI contexts. App extensions are secondary services, bundled within apps, to provide some service. These extensions are invoked in their own process space, typically in the containing application's sandbox. Examples include: - The contents of the action/share menu in apps, which for example lets you share photos onto social media - Audio Units - Photo plugins - Password manager autofill - Content blockers - Custom keyboards - Filesystem providers to have a cloud storage show up in the Files app - Finder synchronization for cloud-synchronized services like OneDrive - All app integrations to extend Siri ExtensionKit (and the underlying ExtensionFoundation) lets a host application define its own extension points, which other applications can provide. I do not believe you can define n-to-m extensions with this interface however; I can create a language server for an iOS text editor and put it in an app, but I can't do that once and support _every_ text editor.