Tube plugs are machined blocks that are put inside an extrusion that is used structurally in place of gussets.
They are threaded so that parts can be bolted into the plug.
Tube plugs are often designed to be compatible with thick wall tubing (typically 1/8” wall), which would leave a small gap between the tube plug and the wall of a thin wall tubing (eg. 1/16” wall or 0.10” wall). A 3D printed sleeve can be used to fill this gap.