we were planning to release our new VTA model (that allows bipolar settings) much sooner but it hasn’t been released so far. It should be a matter of weeks now. The currently available models (Mädler / Kuncel) only allow monopolar simulations (also see their papers).
Sources are usually not needed but could be any independently modeled sources of voltage (e.g. the Medtronic “Interleaved” setting could be modeled with these or the Boston Scientific settings that could later be modeled with our new model).
So in brief: Currently only monopolar stim is possible but bear with us for just another 2-3 weeks for Lead-DBS 2.0 to come out.