Working load limit
Breaking strength divided by a safety factor, commonly 4:1 or 5:1. Side-loading a shackle drastically reduces it.
In practice
Working load limit is the load a component is rated to carry in normal use, arrived at by dividing its breaking strength by a safety factor — commonly 4:1 or 5:1. The gap between WLL and breaking strength is not slack to be spent; it absorbs shock loading, wear, corrosion and the imperfect angles of real recovery work.
The stamped rating assumes a straight, in-line pull. Side-loading a shackle, or loading the tip of a hook rather than its seat, can reduce capacity dramatically — the geometry changes where the force concentrates. This is why rigging is the part of recovery with the highest injury potential and why a strap or shackle that looks adequate can fail: the number on it was never a promise about the way it is being used.