It is important to understand that a FlexPDE script is not a procedural description of the steps to be performed in solving the PDE system.  It is instead a description of the dependencies between various elements of the model.

A parameter defined as P = 10 means that whenever P is used in the script, it represents the constant value 10.

If the parameter is defined as P = 10*X, then whenever P is used in the script, it represents 10 times the value of X at each point of the domain at which the value of P is needed for the solution of the system.  In other words, P will have a distribution of values throughout the domain.

If the parameter is defined as P = 10*U, where U has been declared as a VARIABLE, then whenever P is used in the script, it represents 10 times the current value of U at each point of the domain, and at each stage of the solution process.  That is, the single definition P = 10*U implies repeated re-evaluation as necessary throughout the computation.