The lithium - ion battery supplies power to the electric motor as required and serves to start the engine. (press.bmwgroup.com)
This lithium - ion battery supplies power to the electric motor as soon as it is required to perform an additional drive function or serves to start the combustion engine. (topspeed.com)
Given limited geographic availability, though, it seems most logical that the model is still heavily production limited for some reason — such as lack of battery supply. (cleantechnica.com)