Windows 8.1 keeps in memory the status of an application when the user sends it to background. Although the application’s threads are frozen after about 10 seconds, the application status is kept in memory until the OS runs out of resources.
When the OS needs resources the application could be removed from memory and if this happens the status is lost.
Understanding the possible status of the application and the related events helps to create a better user experience.
e.g.: Serializing the application status to disk when it’s suspended to recover it later.
This diagram describes the life cycle of a Windows 8.1 application.