See Apple's developer document for background.
There is a finite set of lengthy tasks that an application can perform in the background in an Apple document:
For tasks that require longer execution time for implementation, you must request specific permissions to run them in the background without pausing them. On iOS, only certain types of applications are allowed to run in the background:
- Applications that play audio content for the user in the background, for example, an application for a music player.
- Applications that keep users informed of their location, such as a navigation application
- Voice over Internet Protocol (VoIP) Applications
- Press applications to download and process new content.
- Applications regularly receiving updates from external accessories
Applications that implement these services must advertise the services they support and use the system framework to implement relevant aspects of these services. Declaring services allows the system to find out which services you are using, but in some cases these are system frameworks that actually prevent your application from pausing.