PPDS optimizer setting: Validity period of orders: Hard or Soft constraint

hello!

could you please help me with the next point, In PPDS optimizer, tab Order processing, Validity period of orders: regard as hard or regard as soft constraint. It is not really clear what to do with this? I tested both and I found different results, but better results are with Hard constraint. However, from help it says that PPM validity is checked, in our case PPM validity for all orders is until 31.12.9999, so I dont understand why results are different?

if you have some better ideas/understanding on this field and which value to use ?

thank you

Alla