Single layer & Double layer (2 layers glued on each other)
The construction layer in itself is airtight and is about 0.5 mm thick made from mesh and PVC. Some brands simply sold the board in this construction, which called in the industry "single layer"
But others glued a second layer of PVC on the construction layer. This is called in the industry "double layer"
Fusion technology double layer
Instead of gluing on a second layer, the construction layer was simply made thicker, melting more PVC on it. This is called "Fusion technology" or "double layer light" or "double layer fusion"