Ultimate Design Strength.
The aircraft designer starts with a target design strength for a particular weight.
All of the strengths should be consistent. If any structure is too strong then it will be too heavy. If any structure is too weak then this is a design fault and the most likely source of failure.
A wing or engine doesn't fall off. The wings or the engine mounts fail under excessive load.
For safety the ultimate design strength is usually 50% greater than maximum specified for normal operations.
A 3g aircraft should break at 4.5g or a 9g fighter should break at 13.5g.
But beware of the rolling pull out! An up going wing will be loaded up much more than what you feel or what the g meter registers in the fuselage.
Beers all round!