Supplemental Climate Control - Overview: Overview
WARNING: This page is about a different car, the 2017 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.
The climate control system uses the electric booster heater heat the passenger compartment in place of the heater core in low ambient temperatures. Once engine coolant reaches a sufficient temperature to heat the passenger compartment, the climate control system stops using the electric booster heater.