A crud ring is a ring of crud that develops right over the load in the bore when it's been fired. T7 is the worse powder for that happening. The lighter powered 209 muzzleloader primers were developed to help with the crud ring. The crud ring hurts accuracy, and makes loading the next load harder. I won't use T7 powder for that very reason.
I've read some guys say they don't get a crud ring. It's hard to believe. I've had 10 inlines, and they all got a crud ring with T7. I've had ramrod with patches stuck in the crud ring trying to remove it. It's one of the advantages of using BH 209. It doesn't get any crud rings.