Use of fvm vs. fvmb

[jPlasma]

Hello! I'm new to rheoTool and am using it to write OF based solvers with couplings terms. Thank you for the great library!

I have following questions:

1. For coupling terms, can insertEquation function be supplied fvm operators from OF or should only the fvmb operators be used?
2. If fvm operators from OF can be used, when/why are the custom operators defined via fvmb required?

For reference:
Example use case of fvm in insertEquation for coupling terms? I noticed NernstPlanckCoupled.C uses fvm operator:

   // ci_psi
   cps.insertEquation
   (
     ci.name(),
     psi_.name(),
     fvm::laplacian(-ci*cf, psiCorr, "laplacian(elecM)")	 	 	
   );   
 }

Example of fvmb for coupling terms:

// div(U)
cps->insertEquation
(
 p.name(),
 U.name(),
 fvmb::div(-1., U)
);

Thank you!!