Short answer
AISI 304 is often suitable for general stainless steel parts, covers, brackets and indoor equipment. AISI 316 or 316L is usually selected when corrosion resistance is more important, especially around humidity, cleaning agents, chlorides or more demanding industrial environments.
When 304 is usually enough
Choose 304 for general machinery parts, enclosures, guards and components where the environment is controlled and the main requirements are good appearance, cleanability and standard corrosion resistance.
When 316 or 316L makes sense
Choose 316 or 316L for more aggressive environments, parts exposed to frequent washing, outdoor humidity, food-industry equipment or chemical exposure. For welded assemblies, 316L can help reduce sensitization risk.
What to send for a quote
Send the drawing, grade, thickness, quantity, finish, weld locations and any requirement for brushing, passivation or visible surfaces.