Why 2M Cross Axle has Groove? I was sorting axles noticed only 2M has grooves. Why 2M Cross Axle has grooves? Is it for alignment?
I believe it's mostly so that you have something additional to grip on to when removing it from other elements.

With the 3M+ axles, there is more axle available to hold on to so that you pull off a sleeve or joint nicely, but for some, the 2M is too short.