In this instance, a simple question on Reddit had drawn a few quizzical answers, and I thought I could help by going into some depth. And then some more. And then some more again.
It isn't quite as long a post as I would normally make, so I won't charge to my supporting Patreons until the next time around. I hope you find some value in these thoughts!
Casting Checks on The Iron Pact: http://theironpact.com/casting-checks/