question

This may not be supported in future so don't use this throughout your model - better to put it in a user command so if it stops being supported you can change the command to something that is.

Storage.Object rack=current;
int numslots=3;
for (int b=rack.bays.length;b>0;b--){  //bays
    for (int l=rack.bays[b].levels.length;l>0;l--){  //levels
        function_s(rack, "setNumSlots", numslots, [[b,l,1]]);  
    }
}

Sorry - there was a typo in my original post - I've updated it.

But you can see the call to set the slots just uses numeric values for the bay and level - so in your case you would use:

Storage.Object rack=Model.find("Rack1");
int bay=2;
int level=1;
int numslots=5;
function_s(rack,"setNumSlots",numslots,[[bay,level,1]]);