Center of bottom bracket to center (or Top) of top-tube. Depending on the manufacturer. What can get weird is when there is some funky full suspension design the top tube isn't really there. So, you have to depend on the manufacturer to designate a dimension based on a extrapolated location.
Also, generally mountain bikes are X-small, small, medium, large and x-large. In lieu of a 2 cm deviation.
It's always best to not only get the seat-tube dimension but also the top-tube.
For me, the top tube dimension is more significant as you can always raise or lower the saddle, but are more limited with a top tube length.
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger