The typically white W wire is for auxiliary heat; i.e., a second source of heat. (techhive.com)
Prominent among such capital equipment are the water cooled condensers, air cooled condensers, feedwater heaters, component cooling water heat exchangers, steam generators, preheaters, superheaters, and auxiliary heat exchangers. (holtecinternational.com)
The equipment element also includes the service package «Audi connect Vehicle Control», comprising the services vehicle status report, remote control lock / unlock, parking location and the remote control for the optional auxiliary heating. (myelectriccarforums.com)