Sheet Quantity: If above zero, then it will calculate the horizontal and vertical copies for you. If you also manually change Horz Copies or Vert copies with other value will auto-update based on the method selected below
Auto Update (Balanced Grid) : Will calculate the grid size closest to the desired quantity and a 1:1 ratio
Auto Update (Even Grid) : Will calculate the grid size closest to a 1:1 ratio
Auto Update (Exact Quantity) : Will calculate the grid size exactly matching the quantity required