Azure SQL and DTUs

Hi all,

Reasonably easy question. I don't understand the DTU concept that is attached to the Azure SQL database.

Is DTU more like petrol tank on a car where you have certain amount and if you use it you have to fill up or is it more like your car's RPM where you need to keep it under a threshold? I hope that make sense.

For example if each user uses 1 DTU/Second, and I have 10 DTUs available then which one of the following two is correct:

1. Five users can use the site for hours/days/months...because it will always be 5 DTUs/Second
2. Five users can only use the site for 2 seconds...because it adds up to 10 DTUs in that 2 seconds and depletes my reserve

Thank you.

Behdad.