i didnt get it at first either ^ watch the vid and order the bars WITH SLIT threaded means the forktube (the stick ontop of the fork) has threads like a screw and the bars can screw onto it not many bars do so i dont really understand that part but you only need threadless if you use a compression system. a compression system (HIC, ICS, or SCS) hidden internal comp system, iverted comp system, and standard comp system, uses threadless forks it would take me a long while to explain all these so search it. also a slit enables the bars to clamp onto the fork. without a slit the bars wouldnt tighten and they would just slip off. but for ICS, and i believe HIC you need a slit but just order slit or 2' slit on the dropdown box. hope i helped
you are probably just starting. you will learn about the advantages of compression systems as you continue on your scooter journey to scooter enlightenment so dont worry about thoes other choices they will come young grasshoppa