Well the ones that really matter are Schengen, Non-Schengen and USA (also non-Schengen, but special rules).

First Schengen or non-Schengen (which you could also think of as "domestic" and "international" for practical purposes), the idea being that anyone coming from non-Schengen to Schengen will have to pass inbound passport control and anyone going from Schengen to non-Schengen will have to pass outbound passport control. And to make things easier for those who don't have visas to enter Schengen, it is possible to remain airside and make a non-Schengen to non-Schengen connection.

The US, while also non-Schengen, requires an additional, supposedly more thorough security check on inbound flights at the foreign port of embarkation. FRA chose to deal with this problem by grouping all US-bound flights in specific areas of the non-Schengen part of the airport and setting a central security checkpoint for these flights. That's why the USA gates are usually at some dead-end within the terminal - it's easier to isolate.

Now in theory, this system would work quite well. However, the layout of FRA makes the competing objectives and the ever shifting traffic flow (as new countries join Schengen) difficult to deal with. Hence the chaos.

And that assumes nothing is changed in terms of what the US requires, and that other countries don't add or change special rules either.