I'm again new to control line after 45 years. I have a RSM BU style 4 oz. tank which has an pickup tube, a fill/uniflow tube, and an overflow tube. 
I assume the pickup goes to the spraybar. Where does the fill/uniflow tube go to? Where does the overflow tube go to?  I'm thinking the fill/uniflow tube goes to the muffler nipple, but what would you do with the overflow, just route it to the outside of the plane??? Could someone please explain this ?
Thanks for your help....

to elaborate in Alans response -

route the overflow outside the plane - use a piece of fuel tube and use a plug to cap the line - you only need to cap it when you are flying. 

When you are filling - take the cap off, and fill - once fuel comes out of the overflow - re-cap the tube and go.