API600 is the standard for steel gate valves with flanged and butt weld connections; API602 is the standard for steel gate, globe and check valves less than 4 inches.

If the valve size is just within 4 inches, and it's a steel gate valve, that overlap, choose either. Valves that meet more than two standards are called dual- or multiple-rated valves in the US.

Although there is a lot of overlap within 4", the two standards are somewhat different, such as the pressure level and valve material coverage. API600 uses all pressure levels, but the material is relatively small; while API602 has fewer pressure levels, but more materials can be used. API602 gate valve is generally compact, and reduced diameter, the material is forged steel.