Imagine if Apple create a phone, let's call it the iPhone, let's say they get to model number 5. But they want to create a newer faster iphone 5 but keep it in the same case, I think they will probably call it the iPhone 5s. I think the s stands for speedier.

But in seriousness if you are a dealer you should only be able to buy the S model unless your looking at getting one second hand or upgrading a client from the older to the newer model.