I played FE Sacred Stones, Blazing Sword and Blinding Blade (until ch13) before playing FE Heroes. I can't wait for Tana to be out on FEH.
Anyways, I've been checking guides on Gamepress on tiers and stuff and they mention that some heroes have "Low BST".
I may not have looked hard enough but I couldn't find any place explaining what it is. I haven't read about it in League of Legends or other games I've played either.
My best guess is "Burst" but I don't think there's much point in removing just 2 letters.