That is Sales Volume, but its 100M Active Devices, is way slower than what most predicted. Apple took more than 2 years to add 10M Active Devices, on a ~40+M Unit sold during that time, that means 30M are either replacement or, there are quite a high churn rate to Mac. It is possibly the only reason why Apple suddenly come up with iMac Pro, new MacBook Retina etc. It wasn't the sales that matters, it was user leaving its platform. By previous trend and growth at a nearly consistent 20M unit per year, Mac should have at least 120M Active Devices by now, if not more.