Mattresses are made up of different styles and types, from its top layer, down to the core where the innersprings are. These innersprings are actually made of steel and the coils are different for each model or design.
A bonnel coil, for example is the the least expensive type. It’s shaped like an hourglass. Being cheaper than any of the other inner spring coils, this type is prone to wear and damage with eventual use.
A pocket coil is made of springs that are separately encased with their own fabric, foam and whatever other material is in the design. Pocket coils are much more expensive, but this make mattresses even more comfortable and supportive. This is the type of coil used for those mattresses that comes with the zone feature.
A continuous coil is made of a wire system with interconnectng springs. Most hotels have this in their mattresses and what it does best is prevent it from sagging.