[Ikabob]

What kind of plug fits into the extra USB port on the back of the Bike+ Screen and what could I use it for?
Thank you.

 

[SJL]

What kind of plug fits into the extra USB port on the back of the Bike+ Screen and what could I use it for?
Thank you.

It is a USB-C type port. Typically you would use it to connect directly to your network via an ethernet cable for improved streaming stability. Attached is a USB-C to ethernet adapter as the Bike+ does not have a direct ethernet port.
USB-C to ethernet adapter

 

[shoot_the_goalie]

As SJL said. Also you can use a USB-C to power adapter to charge your phone. I use a USB-C to lighting port to charge my iPhone.