So, if you have the kind of panel that's shown in the diagram above, every SECOND breaker on each column in the panel will be connected to the same voltage source. So, if you want to install the wiring to a new 240 volt arc welder, you know that you can put the double breaker to that welder ANYWHERE on the panel and the two sides of the breaker will be connected to opposite voltage sources to give you 240 VAC power.

However, in that post you said that most US breaker panels don't use the AB-AB-AB-AB format, but the AA-BB-AA-BB format instead. What that means is that instead of EVERY breaker position being connected to the opposite voltage source as the one above it or the one below it as it in AB-AB-AB-AB, you have every PAIR of breaker positions being connected to opposite voltage sources. So, BOTH breakers designated as #1 on the panel would be connected to the same voltage source, and ditto for both breaker positions designated as #2, and so on. So, in that case, if you wanted to do the wiring to install a new 240 volt arc welder, you would have to install the double breaker so that it straddled between two numbers. For example, you could install that double breaker at the #2 Lower and #4 Upper breaker locations, or at the #3 Lower and #5 Upper breaker locations so that each side of the double breaker was connected to a different voltage source.