£1.50 fare is for short hop journeys only, which must be completed within 20 minutes, otherwise a single journey is £3.50 and must be completed within 90 minutes from the time of purchase and cannot be used to make a return trip. For your return trip you must tap on again, and the amount charged will be limited to the daily cap.
(from
https://nottinghamcontactless.co.uk/prices)
The only interpretation of that which makes any sense to me is that a short hop is defined as a journey that can be completed in 20 minutes. That makes it very easy: if your second tap is within 20 mins of your first it's a tap out, otherwise it's a tap in for the next journey.
But hang on, no, further up the page it says:
If you are travelling on NET trams wholly within a short hop zone you will also need to Tap Off to get the best fare available.
So, there is a thing called a "short hop zone"? But that isn't referenced anywhere else on the page, there are no "short hop zones" on the zone map and there's no link to explain what it is.
Maybe it's just that if your journey is wholly within one zone it's a short hop, and if it crosses a boundary it isn't? But that makes very little sense, because some of the zones look massive and you could cross the whole thing on a short hop ticket, and yet a one-stop journey that happens to cross a boundary would cost the full fare.
Who on earth writes this nonsense?