The fast algorithm (also weak coupling, one way coupling) calculates electromagnetic sources once and apply them to the thermal model. Reverse interaction (change of electromagnetic properties due to heating) is not taken into account.
The fast algorithm gives good results if the time step is small compared to the heating rate.
The accurate algorithm uses two-way coupling - electromagnetic sources are recalculated according to the temperature field until convergence is reached. The accurate algorithm gives very precise results and is important for highly nonlinear problems.