This is a part that can be easily moved on and off from a rod that holds filament spools.
I've specifically design this because I like the purchase filament refills for cheap, then print my own actual spools.
I love the spool design made by @VkvitV which can be found here: Reusable-Refill-Spool

I love how these spools have clips built in to hold the end of your filament when it's not in use. However, I've ran into problem where these ‘clips’ cause the filament to get stuck, and the printer thinks there's a tangle. 
This centering guide fixes that problem, so you don't have to worry about anything on the sides snagging you filament.

If you want to see more of the models used in my setup, you can find them HERE
 

Many of these models work hand in hand with this design, Such as my spool holder and my top mounted feeding inlet.