Roasting is defined as cooking a solid food with direct heat from a fire.
The Gemara in Chulin (96b) quotes Shmuel that if one roasted a piece of meat with the sciatic nerve (a forbidden part of the animal) still in it, one may eat from the piece of meat until you reach the sciatic nerve itself. The Gemara is contrasting roasting and cooking. When a piece of meat is cooked with the sciatic nerve still in it, the taste of the sciatic nerve travels through the whole piece of meat, forbidding even the outermost edges of the meat. However, roasting does not cause the taste of the sciatic nerve to travel through the whole piece of meat. When roasting, the taste stays where it is. Tosafot and the Rosh explain that although when roasting, the taste of the sciatic nerve does not travel through the whole piece of meat, it does travel a bit, and forbids the amount of a netila (approximately ¾ of an inch).
Tosafot explains that there is a difference between a sciatic nerve, which is a very lean part of the animal, and a part which is fatty. When fatty food gets roasted, the fat acts as a catalyst to spread taste evenly through the entire food being roasted, as opposed to a lean piece of meat where the taste is being transferred through the roasting alone and therefore cannot go more than a netila into the food which is absorbing the taste.
The Shulchan Aruch (יו”ד קה:ד-ה) rules that a fatty food which was forbidden, which touched permitted food while being roasted, would forbid the entire piece of permitted food. However, a forbidden food that is lean, would only forbid a netila. If the permitted food was fatty and the forbidden food was lean, the fat from the permitted food absorbs the taste of the forbidden food and then spreads through the rest of the permitted food, prohibiting it in its entirety.
The Rama is more stringent than the Shulchan Aruch, and rules that we consider all food to be fatty, as we don’t know which level of fatty was required by chazal to be able to spread the taste of a forbidden food. Therefore, we are always stringent and consider the forbidden food as if it went through the entire permitted food, and one needs the permitted food to be 60 times larger than the forbidden food in order to permit the mixture. But, since this is a stringency, one must also remove a netila of the permitted food even if it was 60 times greater than the forbidden food. This is because perhaps the permitted food was not fatty and the taste of the forbidden food never spread through it, and consequently the permitted food never nullified the forbidden food.