As to what premium selection is from what i have researched the "Premium Selection" program the car needs to be near new in condition, low mileage and have full bmw service history with genuine bmw parts to qualify for 'premium selection'.
So in a nusthell the car must be neat clean and in good working order as you could imagine bmw wouldn't wish to offer their warranty program for a vehicle which has a host of problems which will cost them $ in warranty claims.
Also this includes roadside/accident assistance along with a maintenance program that includes free servicing during this time (I am unsure of if parts, fluids & labour are 100% covered?) Also I believe that after the warranty period has expired and you haven't had the service conducted prior to it running out supposedly BMW will still conduct this complimentary service (within reason of course)
As to what is covered, I believe its a 'manufacturers warranty' so its like a new car warranty (unless anyone else can chime in with any further info?)