This is a launch lug or launch guide is used to keep your rockets going straight! This design works best with one inch wide aluminum T-slot extrusions and rockets whose body is 3 inches in diameter (but other sizes will work if you have enough glue). These launch guides mount to the body with epoxy, so they are easy to mount to a model.
The STEP or STP file is uploaded along with a ready to print STL. These STEP files can be imported into several CAD programs (like Fusion 360) and modified as if they were created in that software. This allows for easy remixing. Want to make it work with a different extrusion or a different diameter rocket? Go ahead and import the STEP file as a starting point and get modeling!