From the '07 E90/91 manual:
The heat stored in the engine is
used to heat the passenger compartment,
e.g. while stopped at a
school to pick up a child.
The function can be switched on when the following
conditions are met:
> Up to 15 minutes after the engine has been
> While the engine is at operating temperature
> As long as battery voltage is sufficient
> At an outside temperature below 77oF/25oC
The LED is lit when the function is on.
As of radio readiness, you can set the interior
temperature, the air flow rate and the air distribution