A constant-quantity is a constant value of some physical-quantity, like 3 meters or 55 miles per hour. Constant quantities are distinguished from function-quantities, which map some quantities to other quantities. For example, the velocity of a particle over some range of time would be represented by a function-quantity mapping values of time (which are constant quantities) to velocity vectors (also constant quantities). All real numbers (and numeric tensors of higher order) are constant quantities whose dimension is the identity-dimension (i.e., the so-called 'dimensionless' or dimensionless-quantity).
All constant quantites can be expressed as the product of some dimensionless quantity and a unit of measure. This is what it means to say a quantity `has a magnitude'. For example, 2 meters can be expressed as (* 3 meter), where meter is defined as a unit of measure for length. All units of measure are also constant quantities.
(<=> (Constant-Quantity ?X) (And (Physical-Quantity ?X) (Not (Function-Quantity ?X))))
Assume that quantities have a property like q.unit. Then define two quantities Q1 = (the-quantity 10 centimeters) and Q2 = (the-quantity 0.1 meters). Clearly Q1 = Q2. But (q.unit Q1) = centimeters and (q.unit Q2) = meters. This is a contradiction.
This allows one to commit to the engineering math ontologies without having to handle all the units and dimensions. The theory can include all of normal math as a special case.