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.
Something to consider about cooking it inside the bird vs outside is that cooking it inside the bird will affect your cooking time. It’s especially a consideration when cooking a whole turkey - it’s already difficult to get the dark meat up to temp without drying out the breast.
Having said that, stuffing cooked inside of a bird does taste amazing. You can kind of split the difference by pouring some of the roasting juices onto the dressing and letting it soak those up, but then you don’t get to use them for gravy.