Final answer:
A rating mark on an anchor line indicates the length of line that has been paid out, helping to gauge water depth and ensure secure anchoring.
Step-by-step explanation:
Placing a rating mark on an anchor line, also known as anchor chain or anchor rope, indicates the distance from the anchor to the point on the line. This is important for the person handling the anchor to know how much line has been paid out and how much is remaining. This helps in gauging the depth of the water and ensuring the anchor has enough line out to hold the vessel securely. The marks are often made using different colors or shapes for easy identification at various lengths.