Since you are in the Classic and Vintage forum, that implies that you are looking at understanding frame sizes made in a former era. In America right after the bike boom of 1970, many of our best frames came from England. Those frames were typically sized in inches and were measured from the center of the bottom bracket to the top of the seat tube. The problem is that "the top of the seat tube" is not a very specific defined point that can be identified clearly on any English made frame. It is usually where the seat lug ends right at the very side/center of the seat tube. A frame builder when he is finish filing the seat tube may scalp it a bit more or less. American bicycles like Schwinn and Trek of the same era followed the British example. For example steel Trek frames from the early 80's were 21", 22 1/2", 24", etc. Chicago made Schwinn frames had the added confusion of measuring to the very top of the end of the seat tube while at the same time the top tube came in quite a bit lower than where it would be on a lugged frame. I just measured a 24" 1973 fillet brazed Schwinn Super Sport. From the center of the BB to the very top of the seat tube (cut straight off and not scalloped) is exactly 24". To the top of the top tube is about 23". That is approximately where a British frame would be measured. The distance from the center of the BB to the center of its level top tube is around 58cm. The schwinn way of measuring threw Americans off about frame sizing back then because they might think they needed a bigger frame than what they really needed.
The Italian way to measure a frame was from the center of the BB to the center of a level top tube. This is the way that makes the most sense to a frame builder (like myself) because it is to an exact point that can be precisely replicated in another frame. Most other European frames that are described in centimeters are measured this same way. However there are exceptions like my Italian Masi I got in Italy in 1972. Faliero described his frames in centimeters (and he punched that number on the BB shell) and he measured them center to the very top. My frame is stamped a 58 cm model and he measured to the very tip of the seat lug. When I put it on my fixture and could read exactly what the c-c measurement was, it measured 55.9cm.
Modern sloping top tubes follow different rules and distances in a seat or top tube can be described as "actual" or "effective".