Ah, I think it's like "keep track of 7 items, the 8th might push something else out" ( https://en.m.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two ).

So like mind-map-complexity.
Like, when you are trying to figure out some bullshit OOP inheritance, you are 9 levels deep and have no idea what the original problem was, but you feel like you have made a connection to the solution, but now you have to backtrack everything (or figure out where your last edit was) to try and figure out what that connection actually means and what you have to do now