The question is for Etrian Odyssey 1. As I understand it, the boost adds 5 points to the skill used. But if the skill is already at 10, does boost have an effect in this case?
I tested it out (why I didn't thought about it earlier), and it seems to have an effect, the damage of my attack had increased, even if the skill I use for the attack is maxed out.
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Data-mining of the Untold remake shows that (almost?) every skill actually has data up to Lv.15 (despite capping at 10) just for Boost, so it stands to reason that the original game does as well. – Justin Time - Reinstate Monica Sep 11 '19 at 19:05