Stainless steel plate is bought first by grade. Grade fixes corrosion resistance, strength, heat resistance and price. The austenitic grades (304, 316, 321, 310S) are non-magnetic, easy to form and weld, and make up most of the market. 304 is the general purpose grade; 316 adds molybdenum for chlorides and marine service; 321 and 347 are stabilised for high temperature; 310S is the furnace grade.
Ferritic grades (430, 409, 441) are magnetic, nickel-free and cheaper, used for appliances, exhausts and mild indoor environments. Martensitic grades (410, 420, 440C) harden by heat treatment for blades and wear parts.
Duplex (2205) and super duplex (2507) combine ferrite and austenite for roughly double the strength of 304 with far better resistance to chloride stress corrosion cracking. They are chosen for tanks, pressure vessels, desalination and offshore where 316L is not enough. Super austenitics (904L, 254 SMO) and nickel alloys go beyond that for acids and scrubbers.
The L suffix (304L, 316L) means low carbon, below 0.03 per cent, which prevents sensitisation during welding. For welded plate above about 5 mm, specify the L grade or a dual certified 304/304L plate.