The saveloy dip is a Newcastle sandwich, and the thing that defines it is the dip. A saveloy, the bright, finely-ground, faintly smoky pork sausage, goes into a stottie, the dense, flat, chewy Tyneside roll, and then the whole assembled thing is dipped: the cut roll is plunged into the liquor from the pease pudding pot, or has stock and pan juices ladled over it, until the bread is saturated and dripping. That deliberate soaking is the entire identity of the sandwich and the reason it is not simply a sausage in a bun. Almost every other British sandwich is built to keep the bread from going wet. This one is built to make it wet on purpose, to take the dryness of a stottie and the leanness of a saveloy and bind them with a savoury, slightly sweet liquor into something soft, heavy, and saturated all the way through.
The build works because each part is chosen to survive being drowned. The stottie is the load-bearing decision: a soft white roll would dissolve into mush the moment it met the liquor, but a stottie is baked dense and chewy enough that it absorbs a great deal of liquid and goes rich and yielding rather than disintegrating, which is exactly why this sandwich is made on that bread and effectively cannot be made on any other. The saveloy is poached or steamed soft and laid in whole or split, bringing a smooth, mild, smoky savour that the dip then deepens rather than fights. The classic dressing goes on before the dip and is part of what gets soaked: a thick spread of pease pudding, the smooth split-pea purée, with sage-and-onion stuffing and sometimes mustard, so that when the liquor goes through, it carries the pudding and the stuffing into the bread with it. The result is meant to be eaten at once, over a wrapper, before the saturated stottie loses its structure entirely.
The variations stay inside the dip-and-stottie frame. The sandwich runs with or without the pease pudding and stuffing, with mustard, with extra liquor on request for a wetter result. A ham or a slice of pork can stand in for the saveloy in the same soaked stottie, and the dry version, the same fillings in the same roll left undipped, is the obvious near relative. Each of those deserves its own article rather than being crowded in here.