Both the bottom port and the upper side mount port (A in the image below) (in portrait orientation) are USB Type-C ports. They can be individually used for charging or data transfer, etc. When both ports are connected at the same time, function of the upper side mount port (A) will take precedence . Also ,onlythe side USB Type-C port (A) supports screen output/projection.
The lower side mount port (B) cannot function on its own. When the phone is connected to AeroActive Cooler, Mobile Desktop Dock, or TwinView Dock through the side mount ports, the lower port (B) is used to support the upper port (A) in terms of data transfer between phone and the accessory.
Notice: Please do not insert any connector/plug into the lower side mount port (B) as this will damage the phone and the port.