Each board has two parts. On top is the wear layer, a slice of real hardwood between about 1mm and 6mm thick. Underneath is a core, either several layers of plywood glued at right angles or a high-density fibreboard (HDF) panel.
Because the plywood layers run in opposite directions, they fight each other's movement. The result is a board that expands and contracts far less than a solid plank of the same species.
That stability is what lets engineered go on concrete and over heat. Plywood cores generally handle moisture better than HDF cores, and we steer basement jobs toward them.
Wear Layer Thickness Decides Everything
The wear layer is the only part of an engineered board that can ever be sanded. It decides how long the floor lasts and whether it can be refinished.
Under 2mm, plan on replacing rather than refinishing. At 3mm, one or two careful sands are realistic, and at 4 to 6mm, the floor can be refinished two to four times, close to solid wood.
Retail displays often lead with colour and width. Ask for the wear layer first.