A more thorough explanation:

The Saint Lawrence Seaway Development Corporation (SLSDC) is a company owned by the U.S. Department of Transportation. Its main responsibility is to develop, operate, and maintain a part of the St. Lawrence Seaway that goes from Montreal to Lake Erie. The SLSDC charges tolls at rates that are negotiated with the St. Lawrence Seaway Authority of Canada.

For example, if a ship wants to use the part of the St. Lawrence Seaway that is managed by the SLSDC, it has to pay a toll. The amount of the toll is determined by the SLSDC and the St. Lawrence Seaway Authority of Canada. This toll helps to pay for the maintenance and operation of the seaway.

The SLSDC plays an important role in the transportation of goods between the United States and Canada. By maintaining and operating a part of the St. Lawrence Seaway, the SLSDC helps to ensure that ships can safely and efficiently transport goods between the two countries.