"Rules of the rub" are loose and fancy-free. There's a broad definition as to what rubs are, and within that it's a cook's playground (or the proverbial candy shop) of spices, herbs, and condiments.
A rub is a mixture of herbs, spices, and seasonings liberally applied to coat the outside of meat, poultry, or fish. Salt is always a great starting place for a rub. It helps the rub penetrate, and it rounds out and brings together the flavors of the ingredients. Sugar is also a popular addition to rubs as it caramelizes when exposed to high heat. If adding a sugar, do so sparingly as they burn easily. If using seeds, nuts, dried herbs, or spices, be sure to crush them first to release all their flavor. The only real rule is that there is no right or wrong mixture. It's all a matter of personal preference!
A wet rub has a moist ingredient added to the spices and herbs. Common ingredients added to make a wet rub may include, but are not limited to: mustard, finely chopped garlic, oil, horseradish, and yogurt. Wet rubs are also called pastes, denoting their consistency. They adhere to food more easily than dry rubs.