Total thickness is the overall dimension of the board — for engineered timber, typically 14, 15, 18 or 20 mm. It affects substrate build-up, threshold heights and structural allowance.
Wear layer is a subset of total thickness — the solid hardwood lamella above the plywood substrate. It is the parameter that governs refinishing life and warranty length. A 14 mm board with a 3 mm wear layer and a 20 mm board with a 3 mm wear layer offer the same refinishing performance; the difference in total thickness affects the build-up, not the wear.
Specification implication
A specification that names total thickness alone does not constrain wear layer. Two products at 15 mm total thickness with 2 mm and 4 mm wear layers respectively are dimensionally identical and commercially very different. Both parameters are named in a defensible specification.