The main assembly line builds your car. "Assembly Started" means your car has started down that main assembly line. It takes a day or two to make its way down to the end of it. When my car when through it took a little over a day to move through "Assembly Started."
There are sub-assembly lines that make pieces the main assembly line puts together. Sub-assembly lines make things like dashboards, metal stampings, axel assemblies and such. Some of those assembly lines are not even in Spartanburg; for example, the engines come from Austria or Germany. All these sub-assembly lines are coordinated so what they make ends up in the main assembly line exactly when it is needed.
"In Production" means that those sub-assembly lines are making the parts your car will need. That's why once you are "In Production," you can't make changes to options. It's hard to tell how long a car will be "In Production." It may be that one part your car needs, like the individual Leopard spot upholstery you ordered
, might be out of stock. Your car can't enter "Assembly Started" until all the parts for your car are ready for assembly.
In the end, the schedule on "MyBMW" is doesn't seem to be all that closely linked to what is going on in the factory. I know my dealer called me up to tell my car was already sitting on a truck at the factory and would be delivered to the dealership the next day, but MyBMW showed it was still in quality check. Also, the phone number you call for status didn't always give the same status as MyBMW.
In any case, your dealer may be able to give you a better idea of the actual status of your car.