With traditional geometry having a seat tube that ends at the top of level top tube, the size is from center of BB to top of the seat tube.
Some frames have a seat tube that extends above the top tube about 1/2" where a clamp collar rests. Traditionalists measure these to the top of the top tube, or include the seat collar and add a note saying it includes a 1/2" extension above the top tube.
With the advent of mtn bikes and their sloping top tubes seat tube length alone was less informative. The measurement is still the length of the seat tube (from center of BB), but some other info is helpful, such as measurement to the top tube intersection, but this is often not included.
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