Every headset has an upper and lower stack. I'm pretty sure he is referring only the lower stack height (i.e. crown race plus bottom cup), as he mentions going form 19mm down to 12mm. I don't know of any headsets with a total (upper and lower) stack height of only 19mnm, let alone 12mm, unless we were talking a modern, threadless, integrated headset. Having said that, I don't think I've ever seen a C&V headset with a lower stack height as tall as 19mm, though around 12mm is very common.