Socket Head/Allen Bolt is a bolt with a hexagonal socket in its head that is designed to be used with an Allen wrench. The Allen bolt is integral to the crank and remains in it when the crank is removed from the spindle.
The primary advantage of Allen bolts is that they are perfect if you have a bolt that’s down inside a narrow opening. You can stick the Allen key down into the hole and do up the bolt.
A socket cap screws are machine screws with a cylindrical barrel-shaped head containing a hexagonal socket. The head has a smaller diameter than most other machine screw heads, typically just 1.5 times the nominal thread diameter. Socket cap screws may also be referred to as a socket head cap screw, a cap head socket screw, or a hex socket head screw.
Socket Head/Allen Bolt is often used with the head recessed within a counterbored hole, leaving the end of the head flush with the surface of the part being fastened. This produces a clean appearance and leaves no protrusions that could catch or be damaged, making them popular in sporting equipment such as bicycles.