slot 6

I think it makes the most sense for mons who just need lots of stats, one of them being acc or res. Seara is a good example. Your subs go further if you have predominantly ATK, SPD, HP, and CR subs with a slot 6 ACC than ACC, SPD, HP and CR because you lose a lot of grindable space. It's the same math behind her running a CR slot 4 rune.