There are three primary types of notches - V-shape, U-shape, and square-notch. V-shape spreads less thinset than the same-sized square-notch, while the U-notch spreads out more than the v-notch but less than the square-notch. The major difference is how the thinset lies, affecting tile adhesion. Choosing the right trowel size is crucial for successful tile installation. The trowel size should match the tile size, and the type of notch you use will affect the concentration level of thinset. Also, consider the placement of the tiles and the surface you'll be laying them on to determine the appropriate trowel size.