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All triggers should be on top of events
That is a heavy story. Placing a trigger on top of a top level condition is (especially for beginners) a practice that can (and will) avoid a lot of 'logic' problems.
Yet it is no rule. It is rather a lazy recommendation (i am that lazy too to recommencement this) because it is so difficult to explain.
A trigger runs instantly, when it catches that event that it is designed to catch, no matter where it is coded in the sheet.
If a trigger is buried deep down, as a sub event, it has to look up to find out if it can run. We are used to read the events top down, so a down up logic is hard to grasp, mostly. When the trigger meets an 'else' it gets stuck, by instance. So, the benefits (for beginners, including myself) of always placing a trigger on top beats a lot of problems. Yet, the advanced user surely knows how go along with it, but probably cant explain it well.