The simple answer is “nothing”. A sock is a ventilation duct created in fabric instead of sheet metal or pvc pipe. The technology is widely used across the globe, which means it’s a technology with many names.
In Denmark, where the technology was first commercialized, it would typically be known as fabric ducts, fabric air dispersion, or textile-based ducting. Whereas the same technology is known as air socks or simply socks in North America and textile channels across Europe.
Regardless of which expression is used, fabric ductwork has unique technological characteristics that set the solutions apart from steel ducts, Spiro ducts and other HVAC ducts in sheet metal.