Both standards govern steel gate valves, yet they describe two different products. Ordering the wrong one wastes money at small sizes and is simply impossible at large ones.
API 600 gate valves are cast (typically WCB, WC6, WC9, CF8M), bolted-bonnet, OS&Y valves covering roughly 2" to 60" in Class 150–2500. Casting economically produces the large, complex body shapes, and the standard specifies generous wall thickness, renewable seat rings and hard-faced trim — the workhorse isolation valve of refineries and power plants.
API 602 valves are forged (A105, F11, F22, F304/316) and cover the small-bore range, typically 1/4" to 4", in Class 800 to 2500. Forging gives a dense, porosity-free structure ideal for high pressure and hydrogen or sour service. Bonnets may be bolted, welded, or pressure-seal for the highest ratings; ends are usually socket-weld, threaded or butt-weld.
Whichever standard applies, insist on EN 10204 3.1 material certificates, hydrostatic shell and seat test reports per API 598, and — for sour service — NACE MR0175 compliance. All VTXFLOW gate valves ship with this documentation as standard.