Usually the boxes of screws come with the bottom, more silver bit. But today’s box had the duller grey, bit with a green lap band. No idea if one is better than the other or not.
Edit: Thanks to @0x0@infosec.pub for the wiki article!
Bits like that are used for impact drivers
That reduced diameter acts as a spring to absorb some of the torque being transmitted from the impact driver to the fastener.
The straight bit will not flex or elastically deform, but will fail in torsion without warning. These bits are not “impact rated”, but are meant to be used with hand tools.
TIL!