An instant shift, or instant booking, is a shift that you can be booked into or confirmed for without requiring the prior approval from a facility manager.
An instant shift looks something like this:
So if you've worked at least one uPaged shift at that organisation and you select 
and then confirm, you will be booked for the shift immediately.
If you have not worked a uPaged shift there yet and you select , your application will follow the normal process of requiring review by a facility manager before being accepted or declined.