Square drive recess provides maximum resistance to slippage and stripping.
Self-Drilling No. 2 Point: For .035 to .110 Material.
Self-Drilling No. 3 Point: For .090 to .250 Material.
NOTE: If the drill point is shorter than the total thickness of the material and the thread engages, then the fastener can break or become impossible to drive.
In general, choose a screw length long enough to leave at least 3 threads protruding behind the metal to be fastened.