This definition is used to add or subtract quantities to produce a sum or difference. The quantities are in a list with PLUS or MINUS operators. Hence several quantities can be accumulated either by adding or subtracting as you wish.
Formever maintains unit consistency. So only units of the same dimensionality can be added and subtracted. For example if LotSize is a quantity of acres (dimensionality area) only other area type quantities can be added or subtracted.
Part of this definition is specifying the result type. It must agree in dimensionality with the operands that are being added and subtracted. Formever will automatically convert each of the operands into the specified result type. For example, a quantity of acres could be added to a quantity of square meters and produce the result in square feet.