Apart from the possible amusing replies, my guess it's the marks which the boatyard put on the hull to show the crane/boat lift operators where to place the slings safely.
We are launching tomorrow so I have just checked mine - still there after 3 years.
For modern boats (post 1998) there will be a diagram in the handbook showing where the sling points are - probably also in many earlier boats. If not then as suggested mark where a competent yard places the slings - they usually have a pretty good idea.