That's for road bikes.
The last time I bought a bike, I got thoroughly confused. There used to be just one type of bike and the frame size, IIRC, was measured from the bottom bracket (the bit the pedals turn on) to the top of the tube just under the seat. With mountain bikes, it appears the bottom bracket is higher for improved ground clearance. I was taught you need to straighten the leg as far as possible when pedalling. There's a compromise with being able to put your foot down when stopped. Mountain bikes make this worse.
I thought I wanted a road bike, but when I looked around I realised I didn't want dropped bars and narrow, high pressure tyres. I certainly didn't intend cycling up mountains. 'Sir requires a hybrid' I was told. That appears to be a mountain bike frame (no thank you) with mudguards, rear rack and fat tyres a little less knobbly.
It took me quite a time to find a 'conventional' bike, with the bottom bracket where I wanted it, sensible tyres (700 x 35) and suitable for road and cycle path use. I still got a dozen more gears than I think I need.
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18" mountain bike - big enough? 
18" mountain bike - big enough? 