Pressure vessel

" The 18-8 stainless steels, principally Alloys 304, 304L, and 304H, are available in a wide range of product forms including sheet, strip, and plate. The alloys are covered by a variety of specifications and codes relating to, or regulating, construction or use of equipment manufactured from these alloys for specific conditions. Food and beverage, sanitary, cryogenic, and pressure-containing applications are examples.

Alloy 304 is the standard alloy since AOD technology has made lower carbon levels more easily attainable and economical. Alloy 304L is used for welded products which might be exposed to conditions which could cause intergranular corrosion in service.

Alloy 304H is a modification of Alloy 304 in which the carbon content is controlled to a range of 0.04-0.10 to provide improved high temperature strength to parts exposed to temperatures above 800°F."

http://www.sandmeyersteel.com/304L.html

304L has less carbon in it, resulting in less carbide precipitation at welds, which guards against stress corrosion cracking.

GA Tornado. Concise. I don't think a better answer could have been given in fewer words.

There is no physical difference between the two. The only differnce is limit of carbon percentatge, which imparts difference in their specific use.