The distinction between stuffing and dressing, in spite of ongoing debates about bread bases and mix-ins, can be answered by one simple question: Where does the stuffing go? It's either roasted in the turkey (in which case it's called "stuffing"), or it is cooked in a separate dish (and is called "dressing").
There are pros and cons to both cooking styles. The primary advantages to in-bird stuffing are that it's unbelievably moist and looks super-Thanksgiving-y, but to be safe (with the raw eggs in the stuffing and the raw turkey touching it), you need to cook it to 165 degrees F, which means the bird's going to have to cook to an even higher temperature, which means a potentially dry bird.
If you cook the stuffing outside the turkey, first of all, it's no longer "stuffing" — it's "dressing." Both it and the turkey will cook more reliably, and you can fill the inside of the turkey (loosely) with aromatics if you're going for that Thanksgiving-y turkey-cornucopia look. Also, if you're planning on frying your turkey, or spatchcocking it, you definitely have to cook the stuffing outside the turkey.
But how do you reconcile an evenly cooked bird with not-as-moist dressing? The answer, readers, is turkey stock. Have a little extra stock on hand and drizzle it generously over the dressing before baking. Voila: moist dressing, no turkey cavity required.