SDS Studio™Definition: Under REACH a substance is a chemical element or compound with its impurities and additives, while a mixture is a mixture or solution of two or more substances.
The distinction decides how you classify. A substance is classified on its own data or its harmonised entry; a mixture is classified on test data of the mixture, bridging principles or calculation from its ingredients.
A dilution in water is a mixture. A product such as a cleaning concentrate with ten ingredients is a mixture, even if it is sold under one trade name.
Legal anchor: REACH Article 3(1) defines a substance including the additives needed to preserve its stability and impurities from the manufacturing process, but excluding solvents that can be separated without affecting stability. Classification itself is governed by CLP, not REACH.