Two numbers on the same datasheet, one of which design actually uses — and it isn't the bigger one.
Every material datasheet lists two headline stress values, and it's tempting to treat the larger one as "how strong the part really is." That larger number is the ultimate tensile strength (UTS) — the single highest stress the material reaches before it starts necking down toward fracture. But structural and mechanical design doesn't design to that number. It designs to the yield strength — a lower value that marks the end of purely elastic, fully-reversible deformation and the start of permanent, plastic deformation. The reason isn't caution for its own sake: a part stressed anywhere near its UTS has already yielded, already deformed permanently, and for essentially every engineering purpose is already considered failed— even though it hasn't snapped in two.