When someone says they ride a 54cm bike, is that measurement of the effective to tube length, or the seat tube? Bikeradar had two articles with two different answers
https://www.bikeradar.com/advice/buyers-guides/how-to-measure-a-bike-frame-our-complete-guide-to-sizing-a-bike/
Many manufacturers size road bikes by top tube length.
https://www.bikeradar.com/advice/sizing-and-fit/road-bike-sizing/
Many manufacturers size road bikes by seat tube length (or a nominal seat tube length that imagines the bikes has a horizontal top tube, even if it doesn’t),
by KeyDonut2156