I am using a Roland A-800PRO as a MIDI keyboard controller.  When I look in the menu under Preferences - MIDI - Devices, the unit is listed a few times.  Under Inputs there is: A-PRO MIDI IN; A-PRO 1; and A-PRO 2.  Under Outputs there is: A-PRO MIDI OUT; and A-PRO.
 
Why is there not just a single Input and a single Output?  What do all of these variations mean?
 
Thank you for your help.

General picture you can find on page 66 in the Owner's Manual.
To understand why there are A-PRO 1 and A-PRO 2, read other parts of the manual about "Port 1" and "Port 2".
 
I is good when devices with controlling part and keyboard have more then one virtual port, that way controlling functions have to chance to "leak" into MIDI tracks.