Sir, my name is ***** ***** I am going to make some corrections here, as you have been given incorrect information.
First, you do not have a Honda PowerBoss with a Briggs and Stratton engine. You have a Briggs and Stratton PowerBoss with a Honda engine.
The spark plug you were told is the incorrect plug for this Honda GX390.
The factory plug for it is an NGK BPR6ES.
You can also use a Champion RN9YC.
The RC12YC you were told is a hotter plug than the RN9YC, and can lead to engine over heating due to carbon fouling. The engine does not run hot enough to burn the carbon off of a heat range 12 plug.
A major difference you will notice is that the NGK and the RN9YC use a 13/16" spark plug socket, while the RC12YC uses a 5/8" spark plug socket.
The RC12YC is physically smaller than the RN9YC or the NGK BPR6ES.