The T-nuts provide an option for a mounting point that can be installed in the middle of an extrusion without having to slide the nut all the way in from the end. This allows for faster installation as well as installation after the structure has been fully constructed, it does require a bit of finesse to install properly though. Have a look at the T-Slot primer in the resources section for more information on installation. These nuts are suitable for use with M4 bolts.