How does Crit Chance work?

Hi guys. I hope someone can help out:

Apparently in POE1 crit chance is rolled for per action and not per hit. I also read someone saying "bonestorm can (will often) crit 2-4 times". Which leads me to assume that each phase of a channel has its own crit roll. Is that correct? So does each phase of Flame blast then also have its own crit roll? Same for Firewall damage instances?

Trying to synergize crit recency with crit chance on second weapon set. My bread and butter and only damage right now is Flameblast and Im trying to see if I go phys or frost for my second set. Because if Bonestorm really calculates per instance you have an 80% crit chance on a full channel out of the box and Stormcharged would almost self sustain on my Flameblast build (50% per cast and i can cast >1/s).

If a skill hits, that hit has a chance to crit unless otherwise stated in the gem. Like in the case of Flameblast the hit happens whenever you release it and crit is rolled then.

If you cast Arc and the skill crits, then all of its chains will also crit.

For multi hit spells I assume its the same behavior. Maybe bonestorm's daggers are rolled individually before they're fired, maybe someone else will know.

thanks a lot that helped me out already. i think it will be tied per instance of damage then. 1 aoe = 1 instance of damage, 1 projectile = 1 instance of damage (bonestorm specifically states it fires up to 20 projectiles). so the damage itself rolls for crit not the cast, action or hit (which matches with your statement about chain lightning).

On a side node the weapon set mechanic is really neat, im level 77 or something and havent used it yet but even if you would just use it to reach sockets, its a nice power spike. Now it allows me to slot comet or bonestorm back in for burst or crit focused shenanigans.

Bonestorm is a sustained skill, you can hover the tag to read what it means.

"

Sustained Skills
Sustained skills cause a large number of Hits over a period of time. They roll for Critical Hits independently each time they damage enemies, rather than rolling once for the entire skill as other skills do.

i actually missed that. well then its 1 per cast unless its specified with that skill tag.

additionally, i realized after looking if i want to go ice or phys for my second loadout that i can only use 1 loadout in demon form... i mean might be obvious but i was really excited getting more spells back in. ive been pressing 2 buttons pretty much since maps started and im about to reach t8.