MacBook Pro > MacBook Air > Macbook
iPad Pro > iPad Air > iPad ( The Mini actually fits into the Mid Range )
It is similar on iPhone, where I expect Apple to drop the number from naming scheme and just go with
iPhone S > iPhone R > iPhone
Basically Apple is transition all of their product line into three different segment, each with different display size and prices. And I guess these transition will make a lot more sense once the 2019 and 2020 product are out.