This is a clip for holding wire management tubing (also called split loom conduit). This was designed initially to attach to the for the Deltaprintr's 15mm aluminum extrusions. Presumably it could also work for other uses cases. The type of tubing I used with it was approximately 1/2" or 12.7mm outer diameter. The inner diameter is approximately 3/8" or 10mm. Here's a link to some similar tubing. I bought mine at Home Depot but couldn't find the exact same thing on their website.