Halakhah for Chullin 209:13

If someone is about to say birkat ha-mazon, he is required to wash his hands first, and then say the blessing “al rehitzat yada’im;” this washing is an obligation. And so they said, “Mayim rishonim is a mitzvah, mayim ahronim an obligation (hovah), washing during the meal optional.”¹⁶⁶B. Hullin 105a. An obligation – hovah – is a stronger requirement than a mitzvah. And when they said “during the meal,” they meant it is optional between one cooked dish and another. But between a cooked dish and cheese, it is an obligation.¹⁶⁷Ibid., 105b.

Final waters (to wash hands at the end of the meal) are an obligation. And it needs to be cold water and that if fall into a vessel or into anything that separates between it and the ground, such as shavings and that which is similar to them. And one who did not eat anything [messy] and did not touch salt during the meal does not require it.