Influence diagram: when we use this statment engines{end+1} = belprop_inf_engine(limid);

[sammadikhouloud]

I read the different examples that create an influence diagram (BNT=> Limids), but I don' understand when I must use :
engines{end+1} = belprop_inf_engine(limid) ?
what means this statement?
exact = [1 2];
approx = 3;
please help me