The "TELE-ingang" is just an input to set the setpoint to a preprogrammed value, nothing special. Before you buy anything make sure you've had a look at them in real life: The touch screen version looked very appealing to me too at first, but when I saw it in real life it looked very cheap to me. It's polar white and it seems to be made of the cheapest plastic they could find.
When I was still considering an ATAG boiler I had a glance at their thermostat too, the BrainQ, but that thing is just butt ugly. I really liked the Nefit Module thermostat. It's got graphic back-lit display, capable of plotting setpoint and measure temperature charts. Both have a digital input to control it externally, but both use a proprietary protocol, my main reason for not buying any of their equipment.
I eventually decided to get a Remeha Celcia 20 thermostat. It's not the prettiest pencil in the box but my reasons for buying it were:<ul><li>It uses the OpenTherm protocol</li><li>It has a digital input, programmable to perform a wide variety of tasks when triggered</li><li>It can be programmed with switch points instead of temperature periods</li><li>It can be fully controlled through the serial port of the optional Remeha GateWay</li></ul>The third point was important to me becuase I want to be able to define a scheme which only switches <i>off</i> the heating system without ever switching it on. So the user sets the setpoint to 20 degrees when he comes home and the scheme switches it to 17 degrees after 12 o'clock.
Good luck!
. A good starting point for this thermostat in general would be the users manual: