This is defined as 'The amount an insurance company will cover in a safe/cabinet overnight'. If this is of importance, it is worthwhile to check with your insurer what level of cover they will offer.
Safes are available in various qualities and quality is defined as 'how resistant to attack'. The higher the level of cash or valuables being left in the safe, the better the quality. As quality increases, then generally the body of the safe becomes thicker as does the door. This is due to more barrier materials being used in construction to impede attack.
Insurers categorise safes in relation to the protection offered. Typically insurers multiply the cash rating given to a safe by ten to give the level of cover for valuables.