Why should there be more than one plimsoll line?

There is ONLY one legal Plimsoll Line, the International load line, which is a marking on a ship's side showing the limit of legal submersion when loaded with cargo under various sea conditions.

However, depending on the temperature and salinity of the water on which the ship is floating the position of this line (which represents the weight of water displaced by the hull) will vary. So the line is graduated into:-

The American Bureau of Shipping (AB) Line/ Summer Load line (S)

The Tropical Freshwater (TF) line

The freshwater Load line (F)

The tropical Zones load line (T)

The Winter Load Line (W)

Winter North Atlantic Load Line (WNA)

Thus if a ship were loaded to the TF line in tropical Freshwater then moved to the North Atlantic in winter with the same cargo loaded, it should float at the correct line in both conditions.