source term define
 

In UDF manual for source term define:"Your UDF will need to compute the real source term only for a single cell and return the value to the solver, but you have the choice of setting the implicit term dS[eqn] to dS/d_phi, or forcing the explicit solution of the source term by setting it equal to 0.0.". My question is in what situation, we can set dS to 0.0 and how it will affect the results? Thanks in advance.

BR KK