A STANDARD-QUANTITY is everywhere continuous, has a piecewise continuous derivative, and a dimension. Quantities in QPT and QPC are standard-quantities. In QSIM, the derivatives are continuous as well within a behavior.
(<=> (Standard-Quantity ?X) (And (Everywhere-Continuous-Quantity ?X) (Value-Type ?X D/Dt Piecewise-Continuous-Quantity) (Value-Cardinality ?X D/Dt 1)))