VB relevance in alternative materials

Hello

I would like to know the relevance of parameter VB (MRP type) when we have alternative materials with a different usage probability inside a BOM. Currently we have three materials inside a BOM and just one is used (so probability = 100%). All of them have MRP type VB in the material master so we have purchase requisitions for the three materials even if its usage probability is zero in the BOM. Is it the normal behaviour? Is there a way of avoiding it in order to just have requisitions for the material with probability not equal to zero ?

Thanks