Image: Rebel Wolves / Bandai NamcoOfficial screenshot from Rebel Wolves / Bandai Namco.
Sustained Focus is a Swordmastery perk that gives Coen more Activation Charges. More charges means more Active Abilities per fight, so it helps every build.
It raises how many Activation Charges you can bank. Rank 2 also lets you open a fight with a charge ready, so you can use an ability right away.
Yes for almost any build. Charges fuel Swordmastery, Witchcraft and Vampirism abilities, and Swordmastery is the tree guides say generates them best.