The two numbers stamped on a metric bolt head are a strength specification in code. Under ISO 898-1, the property-class number encodes specified minimums: an 8.8 bolt has a tensile strength of roughly 800 MPa and yields at about 80% of it — that is what the “.8” means — with the standard classes spanning 4.6 through 12.9. Inch fasteners carry the same information differently: SAE grades mark strength with radial dashes on the head, none for Grade 2, three for Grade 5, six for Grade 8.
The class is what turns geometry into load capacity. It supplies the proof strength Sp and yield strength Sy a joint calculation runs on — class 8.8 at M16 means Sp = 580 MPa and Sy = 640 MPa, so with the 156.7 mm² tensile stress area the proof load is 90,870 N, and every preload target is quoted as a fraction of that figure.
Published tightening-torque charts are the class tables in disguise: each cell is T = K·F·d computed with the chart assumptions K = 0.20 (plain, dry steel threads) and preload at 75% of the class’s proof load — which is why the same M-size gets three torque columns for classes 8.8, 10.9, and 12.9, and why those dry values over-tension a lubricated or zinc-plated bolt, which reaches the same preload at 15–25% less torque.