Sorry if this is common knowledge, but I've seen some conflicting reports of how poise works in this game and haven't come across a post specifically about it yet.
Rather than being tied to which kind of armor you are wearing, your poise is determined by your toughness value. Similar to how agility works, there are different tiers that change how you stagger when you get hit by enemies. There are 3 main tiers for toughness. Under 100 (yellow) which you will only really have if you leave armor slots empty, 100-199 (green) which most people probably fall into right now, and 200+ (blue) that can usually only be achieved by wearing at least one piece of heavy armor. I was able to test 400+ toughness, so it doesn't look like there are any tiers above blue. The differences between yellow and green poise are pretty subtle, but it feels like you get locked down for just a tiny bit longer when you get hit if you have yellow poise. Blue tier poise is a much more noticeable change, however. You still get staggered when you are hit normally, but ALL of your attacks gain hyper-armor, meaning that when you go to attack, that attack will be completed no matter what, even if you get hit mid-swing.
Usually you have to wear heavy armor to get up above 200 toughness, but there are some interesting ways to accomplish that without investing too heavily into stamina.
TL;DR: Poise is tied to the toughness stat, and any number over 200 gives you hyper-armor on your attacks.