There are two common scenarios when you would keep the thermostat's internal sensor set to a percentage instead of setting it to disabled:
1. If you want to use the thermostat’s internal sensor in combination with the external sensor, where both are being averaged to maintain comfort, then you will want to set the master thermostat to a weighted percentage.
2. If you weighted the external sensor to be the primary sensor (set to 100%), but if the external sensor fails, you want the thermostat to automatically re-active its internal sensor as the control to temperature, then you will want to set the master thermostat to 0%.
Note: Setting a Master Thermostat’s internal sensor to Disable means it will NEVER be used, even if the external sensor fails. This is common in situations where the master thermostat is sensing a temperature that has no relevance to the actual room temperature being maintained.