16mm polycarbonate sheeting is the lightest available triplewall polycarbonate sheeting. This means rather than being formed of two layers and one hollow chamber they have three walls and two hollow chambers. This increases their insulative properties and their weight, obviously. This makes them more acceptable for use in larger freestanding structures than the doublewall sheets which are too light and vulnerable to the wind. 

16mm sheeting is still appropriate for larger versions of the structures that would use 10mm, so large gazebos, large pavilions and so forth. But it can also be used for wholly different ones. For example, 16mm sheeting is perfect for carports. Carports are essentially open-air garages that protect your cars from the elements.