Ultimate tensile strength is the headline of a fastener’s marking: in the ISO 898-1 property-class code, the first number states the tensile strength in hundreds of MPa and the second the yield fraction, so class 8.8 means roughly 800 MPa ultimate with yield at about 80% of it. These are specified minimums the class guarantees, spanning 4.6 up to 12.9 — and where a bolt actually reaches that stress is the thread region, which is why calculations use the tensile stress area At = (π/4)(d − 0.9382·p)², a reduced area that correlates with measured tensile strength rather than the nominal shank area.
In service a bolt is meant to live well below ultimate, and the class supplies the whole strength ladder for that bookkeeping: an M16 class 8.8 carries a proof strength of 580 MPa and a yield of 640 MPa under its nominal 800-class ultimate, and preload targets are quoted as fractions of the proof load (At·Sp) precisely so the working stress stays under every rung — the standard 75%-of-proof target leaves the example bolt at 435 MPa, about 68% of yield.
Outside the fastener world, tensile figures appear in property tables mainly where yield is not the natural measure: general engineering charts quote yield minimums for the structural metals but fall back to tensile values for materials like virgin PTFE (20–30 MPa), and print ranges wherever grade and formulation genuinely move the number.