A more thorough explanation:

A stiffening note is a permit issued by a customs collector to the ship's master. This permit allows the ship to receive and load heavy goods that are necessary to balance the weight of the vessel before the inward-bound cargo has been completely unloaded.

For example, if a ship is carrying a heavy load of cargo, it may need to take on additional weight to maintain its balance. The stiffening note allows the ship to load heavy goods, such as ballast, to help balance the weight of the cargo.

Another example would be if a ship is traveling in rough seas and needs to stabilize its movement. The ship's master may request a stiffening note to load heavy goods to help stabilize the ship.

In summary, a stiffening note is a permit that allows a ship to load heavy goods to balance its weight and maintain stability during travel.