Dislikedyeah the key concept is: price remains shortly at a demand/supply zone and then shoots off in one direction, if south then it's a supply zone if north it is a demand zone. Expectation is that this area where price has been before is now noticed by the experts and a lot of orders get stacked there, so if price retests that level, the professionals get in and cause another move from that area.
attached is an example (at least I believe it is an example) Please, anybody with more experience with seiden's material correct me if I am wrong. the orange...Ignored
With respect to supply demand and S/R i have seen these terms used interchangeably. Resistance means when supply is expected to overcome deman and vice versa.
When you say: "For me demand / supply zones can be the same as S/R levels but don't have to be. I think if they match up it is a deadly combination. (deadly meaning high probability)"
Correct me if wrong .... so you are saying is : A clearly defined level + previous High Momentum reaction on that level = Deadly Combination aka high probability.