A Principal for the purpose of this question is the one who is being represented by an agent. The agent is hired by the principal to do work for them after which the agent will be compensated for their efforts.
In this scenario, Diamonds and Gold LLC are the principal because they hired Elle who therefore becomes their agent, to buy gems and precious metals on their behalf.
What else that is needed for a powerboat to be operated in compliance with the law aside from the registration number is the display of valid state registration decal while boating, although, each State has different specifications on what powerboats to be registered.
