The 690 fuel system is basically one giant interconnected tank (actually, it's something like 22 different bladders, but functionally it is all one tank).
The close in ("center") fuel cap on the right side lets you fill what we always called the "center tank" and gives you around 1800-2000 pounds. That happens to be a nice fuel quantity because you can more or less fill the seats with that amount, and still fly for 3 hours.
The wing caps let you fill the airplane the rest of the way up.
Be careful never to open the center cap once you have filled it from the wing, as it will massively overflow.
