Safe Working Load is specified by the manufacturer and is generally defined as “the mass or force that a piece of lifting equipment, lifting device or accessory can safely use to lift, suspend, or lower a mass without fear of breaking.”
It’s calculated by dividing the Minimum Breaking Strength (MBS) also known as Minimum Breaking Load (MBL) of the material by a safety factor, which is assigned depending on type and use of the equipment and tends to usually range from 4 to 6, except if failure would cause risk to someone’s life, in which case the safety factor is normally 10. For example, if the Minimum Breaking Strength of the material is 200kg and the safety factor is 5, it means the SWL is 40kg.
The safety factor tends to vary between fixings, but for example for Corefix fixings the safety factor is 7 for pull out (tensile) force and 4 for shear force (more on these below!)
Great, so what does all that mean? In a nutshell, it means that the Safe Working Load describes a force that is much less than the force needed to make the fixing break, but it’s the most weight it can hold safely.