What is the "Vertical Line Number" in Genie editor?

I am trying to add a new tech to be researched in a custom building for a mod. So under Tech Trees > Techs tab of the Genie editor there are entries that connect tech research to buildings (via tech index and upper building) if I understand correctly.

But that tab page also requires the “Vertical Line #” which doesn’t seem mandatory so doesn’t have any associated tooltip info. Looking at the other techs declarations I could not find any correlation between their vertical line number and anything else in the editor with that index. What am I missing?