Honestly for me, it’s simply a matter of how it’s cooked. If it’s “stuffed” in the bird, it’s stuffing. If it’s in a dish on the side, it’s dressing. But I do reserve that stuffing can be pulled out and served on the side. And you can also eat stuffing without ever “stuffing” it. Stove Top Cornbread Stuffing is my weakness and I use it as a quick and easy side often with a protein. Or when making crab cakes.
Living in the mid-Atlantic region of the US, it is always stuffing. Dressing is what you put on salads. I have never heard stuffing called dressing around here.