Gear Slings otherwise known as Bandoliers are padded slings used to carry climbing protection. There are 3 main types of Gear Sling.
A standard Bandolier is worn over your shoulder and your karabiners clipped to it underneath your arm.
An Adjustable Bandolier is used in exactly the same way as the Standard type but its length is adjustable to enable a precise fit.
A Big wall Bandolier generally has two carrying Slings, one under each arm and two shoulder straps. They are normally worn like a rucsac and are designed for use when you are carrying a large amount of equipment.
Gear Slings are generally used to organize your gear and take the weight off your harness, and are especially useful if you are climbing with a large amount of gear on long climbs. They are also useful if you are climbing with a rucsac on and your harness gear loops are obstructed. They will also speed up exchanging the rack with your partner when you are swapping leads on multi pitch climbs whether rock climbing or mountaineering.