The Plimsoll Mark, also known as the load line, was created by William Plimsoll to indicate the safe limit for a ship's draft based on its type, reserve buoyancy, and intended voyage route. The marks start at the deck line and indicate the maximum safe waterline for various seasons and locations. Factors like fresh water allowance, which accounts for differences in the density of fresh water versus salt water, are considered when determining a ship's Plimsoll Mark.