Dosing is about replenishing minerals in the water. Depending on the corals and fish they will remove minerals at different rates. Sps corals have thick skeletons of calcium so you need to replensh the calcium. Clams and crabs are the same.
water tests will show you how much is being used and let you know how much to replenish.
Simple a b cal and alk are the easiest and really hard to over dose. The same for most off the shelf mag. Iodine is also an important. none