A ORC is part of the tractor, while a slip clutch is part of an implement. The purpose of a slip clutch is to reduce chances of damage to the PTO drive and implement if it becomes jammed. Incidentally, some people claim that slip clutches are difficult to keep in adjustment, and shear pins are actually more reliable. Shear pins are soft metal bolts that hold PTO shaft couplers together. The pins break if too much force is applied to the shaft.
Shear pins come in all sorts of sizes, lengths and grades.
When buying a used implement that uses shear pins, it's a good idea to determine the proper pin. It's not uncommon to find a regular hard bolt in place of a proper shear pin.