An "options" attribute is added to PushSubscription objects that the PushManager.subscribe(...) and getSubscription() promises resolve to. It reflects the options passed in when subscribing. This allows web developers to see what applicationServerKey each PushSubscription is associated with, and hence what key their server will need to present when sending messages. Previously web developers with more than one applicationServerKey had to keep track of this separately (e.g. in IndexedDB).
Docs: https://developers.google.com/web/updates/2016/09/options-of-a-pushsubscription