Devs can remove, tag, group notifications or replace with a new one. They can also set when the notification expires. They can also enable Ghoast toast notifications, which will appear only in notification center.There is a limit of 20 notifications per app and notification for last 7 days or less will be shown. Users will be able to tap on, remove one, a group of or all the notifications.
You can read the most comprehensive Windows Phone 8.1 changelog by going to our Windows Phone 8.1 page.
Thanks WPFan for the tip.