I did not receive any instructions with this unit, asking what is the extra PCB that came with this loco used for.
After looking at the online reference for that locomotive, I assume it's the DCC decoder mentioned on page 5. It's not required for operation if you use DC control.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/81294.pdf
Flare, the diagrams you link are to the older outside frame 2-8-0, not the C-19.
There are two plug-in boards that come with the C-19. They get plugged into "the socket" which is in the tender. The first (which comes installed from the factory) is what's called the "dummy plug." It allows the loco to run on analog DC track power. The second plug--the one of near identical size but with a ton of wires sticking off either end--is the interface board which can be used to wire in any 3rd-party onboard throttle control. (DCC decoder, battery R/C throttle, etc.) You would use this when the particular throttle you wish to use does not come in a "plug-and-play" format.
Later,
K