In terms of pilots, what is an ace, and can you name all the aces from the vietnam war?

Aces were the top pilots of each nation who had downed at least five enemy aircraft during aerial combat.

Air Force Vietnam War aces
3 total
Capt. Charles DeBellevue * 6
Capt. Jeffrey S. Feinstein * 5
Capt. R. Stephen Ritchie 5
*Weapons systems officer ace

Aces

That is correct. The source I have only mentions Capt. R. Stephen Ritchie. I wonder what exactly weapons system officer ace means, and why it would cause Capt. Charles DeBellevue and Capt. Jeffrey S. Feinstein not to be included on the list.