The shackle is the U shaped locking bolt that locks the padlock. This can be constructed in a variety of materials that range in security level.

Non Hardened Shackle
Used in very low security applications mainly with low security laminated steel locks.
These shackles can be easily cut with bolt cutters.

Hardened Shackle
Shackles are heat tempered to make the shackle harder. These shackles are harder to cut providing added security.

Hardened Molybdenum Alloy shackle
Hardened Alloy Shackles are made from a superior quality of steel specifically designed to provide maximum hardness when heat treated. These shackles provide the maximum security possible against cutting with bolt cutters. .