What is a 'drop axle' or lift axle (podizna osovina) on a multi-axle heavy truck or trailer, and when should it be lowered?

1. An auxiliary axle that can be pneumatically raised when empty to reduce tire wear and toll costs, and must be lowered when loaded to distribute weight and prevent axle overload
2. A spare axle carried on the roof of the cabin
3. An axle designed to fall off in an emergency collision
4. An axle used exclusively when reversing