Charge Assist....is it worth it?
I recently took a training class at our Trane wholesaler and that's exactly what it does - you hook this up to the system, hook the cable to the control board, hook your gauge set up with the bottle in vapor mode, and then allow the system to charge itself. It basically calculates subcooling and adjusts from there. It must be used with the communicating models (I think only the 20i, although the 16i might do this as well), since those have all the sensors in place to determine the correct values.
A neat toy, but not really useful - and if you have a good thermometer and a good set of gauges, then just another toy to get lost in the toolbox!