For superior corrosion resistance, Stainless Steel is the preferred option. This is because the coating of a galvanized fastener is usually compromised during installation, exposing the carbon steel core to corrosive elements. Even the best galvanized fastener suffers severe corrosion after exposure to harsh conditions.
Lag screws are used to lag together lumber framing, to lag machinery feet to wood floors, and for other heavy carpentry applications.
These fasteners are "screws" according to the Machinery's Handbook criteria, and the obsolescent term "lag bolt" has been replaced by "lag screw" in the Handbook. However, to many tradesmen, they are "bolts", because they are large, with hex or square heads.