Porridge oats are more finely milled than the jumbo oats. Broadly speaking, they are produced by taking the jumbo oats (which are already rolled) and rolling them a bit more. These porridge oats are a bit more fine and have flakey bits and powdery bits, which means that they absorb more milk and go softer. This makes them ideal for porridge, but there is no rule saying that you cannot have them in muesli, which is what we do.
You can also use these porridge oats in cooking, e.g. flapjacks and rough biscuits.