Alpha Climatic Installation & User's Instructions page 17

The primary flow temperature might not reach the temperature set by the user when
in operation because the programmer will control the boiler output to give optimum
performance to reach the required room temperature setting.
c. External weather compensation probe (optional)
When an optional external weather sensor is fitted the CH temperature control dial
will no longer adjust by degrees °C, instead the display will show a scale of 1 to 9.
17
Outside Air Temperature