The setback temperature being the temperature you never want the house to fall below when it is "out of hours" of operation, with the "On" temperature(s) being the targets you want when you would like to be warm.
The "setback" temp automatically becomes effective during the times that no specific temperature has been programmed.
The Set Temperature at the top will allow you to make a non-scheduled temperature change which will immediately take effect and last until the next change time you have in your schedule.
So if you were to come home early, for example and set your temperature to 21C, it would then immediately target 21C and then revert to your normal schedule whenever you have a heating block later in the day.
The system will switch the heating on when the temperature drops below 0.5C, and then switch it on when the temperature rises above 0.5C. So in your instance your thermostat will switch off when the temperature rises above 16.5C.
The schedules for heating products are stored in the peripheral products themselves, so are not reliant on the Link being powered or an app being open. A record of the schedules are stored on the server and the apps. If you make any changes on one app, they will be stored immediately to the server, then you can obtain these changes by doing a "Pull to Refresh" on the second device, where the settings will sync.