Yield strength is a material property describing the highest internal stress in a material that an item could achieve without permanently deforming.
Ultimate strength describes the internal stresses at a load that would cause failure.
Almost all codes require an engineer to use the yield strength for design except in cases like fall protection. In this scenario, it is common to see ultimate strength used in conjunction with a high loading scenario and the requirement to re-inspect after it's been loaded. It is important to keep in mind that many high tensile materials such as quenched, tempered or high carbon steels have a narrow band between yield, ultimate and transition causing permanent bending to breaking very quickly.
The breaking load being five times greater than the working load is commonly called a safety factor. Safety factors are applied to devices in order to protect everyone from unforeseen events. They are usually derived from assessing the risk to human life and the confidence in a loading scenario. For instance it is common in below the hook lifting devices to have a 3:1 safety factor on the yield stress because the risk to human life is high and the loading is moderately controlled.
Chain Blocks are also used on construction sites where they can lift loads from the higher levels, in assembly line factories to lift items to and from the belt and sometimes even to winch cars from a treacherous terrain.
Chain Blocks come in a variety of different capacities making them suitable for a wide range of operations.