Buy Item sourcing Rule

Summary: We source same item from multiple suppliers so we defined sourcing rules with Supplier1, 2, 3 with Rank 1, Rank2, Rank3. How planning decide the supplier in constraint Vs Unconstraint plan? I was expecting to see rank1 supplier with planned order which we can change to Rank2 or Rank3 but thats not the way its working...sometime its defaulting to Rank2 so looking for Logic.

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