Supplementary liability insurance provides cover if you are involved in an accident while driving a rented car and cause damage to a third-party such by injuring them with the rental vehicle or causing physical damage to their property.
Many countries throughout the world include supplementary liability insurance cover as standard, but in the USA, Canada, the Caribbean, South or Central America, the amount of cover tends to be quite low. Supplementary liability insurance is often offered as a top up to ensure you have sufficient cover. This can be bought at the rental desk or it can be purchased as an add-on to many standard car hire excess insurance policies.
Some companies will also offer supplementary liability insurance as a standalone product. If you plan to rent a vehicle in the USA, Canada, the Caribbean, South or Central America, it is definitely worth looking into purchasing this cover. Supplementary liability insurance is often referred to as SLI cover.