What happens after app is closed

I was hoping to understand abit more about what happens when you exit an app and return to the main screen and what role appID plays in functions like the one below, if any.

 1. @param {boolean} isOn - True if the GPS should be on, false if not
     * @param {any} appID - A string with the app's name in, used to ensure one app can't turn off something another app is using
     * @returns {boolean} Is the GPS on?
     * @url http://www.espruino.com/Reference#l_Bangle_setGPSPower
     */
    static setGPSPower(isOn: ShortBoolean, appID: string): boolean;

1. Is there an "onExit" type event you can subscribe to so can do things like saving app state before it closes?

2. If I don't set the app id is it auto injected?

3. How uptodate are the comments on functions like "setGPSPower"? I had assumed that once you exit an app to the main menu everything is reset to how it was before you opened the app. The comments kinda hint that multiple apps could be open at the same time. In what scenario would this happen? Widget?