I think Aruparking was not sure at the beginning about the hours they want to charge for parking. They made several changes.
If you park at your usual parking spot, you should as well see a large, blue, square parking sign. Usually, they write down the times when they charge for parking, like 7 a.m. to 7 p.m.
When you get your parking receipt (the little slip from the machine), the time you paid for is written on it. If for any reason you need to add parking time, you can do so at any other parking machine, just take the little parking slip with you where you can as well see your parking lot number.
As all parking machines are electronically connected to tablets which the parking guards have, it's not necessary to leave the parking slip in the car. They can see electronically on their tablets who paid and who's parking time has expired.
Hope that helps.