They all possess the same nominal chromium and nickel contents and thus possess the same corrosion resistance, ease of fabrication and weldability.
The carbon content of 304H (UNS S30409) is restricted to 0.04-0.10%, which provides optimal high temperature strength.
The carbon content of 304L (UNS 30403) is restricted to a maximum of 0.03%, which prevents sensitization during welding.
Sensitization is the formation of chromium carbides along grain boundaries when a stainless steel is exposed to temperatures in the range of ~900 °F (480 °C)–1,500 °F (820 °C). The subsequent formation of chromium carbide results in reduced corrosion resistance along the grain boundary leaving the stainless steel susceptible to unanticipated corrosion in an environment where 304 would be expected to be corrosion resistant. This grain boundary corrosive attack is known as intergranular corrosion.
The carbon content of 304 (UNS 30400) is restricted to a maximum of 0.08%. Thus 304 is not useful for corrosive applications where welding is required such as tanks and pipes where corrosive solutions are involved, thus, 304L is preferred. And its lack of a minimum carbon content is not ideal for high temperature applications where optimal strength is required, thus, 304H is preferred. Thus 304 is typically restricted to bars that will be machined into components where welding is not required or thin sheets that are formed in articles such as kitchen sinks or cookware that are also not welded.
Carbon content has a strong influence on room temperature strength and thus the specified minimum tensile properties of 304L are 5 kilopounds per square inch (34 MPa) lower than for 304. However, nitrogen also has a strong influence on room temperature strength and a tiny addition of nitrogen produces 304L with the same tensile strength as 304. Thus, practically all 304L is produced as dual certified 304/304L, meaning it meets the minimum carbon content of 304L and also meets the minimum tensile strength of 304.