SR based IV field in Item details of Contract

Hi experts,

I have issue in contract creation,I am creating a Material contract and Service contract below are observations-

1. For Service contract “SR based IV” field in the item details is ticked independent of the settings in vendor master.

   2. In case of   Material contract “SR based IV” field is untick .

I checked with both settings in vendor master purchasing view Tick and Untick.

I checked for item category D but this SR based IV field does not maintained in item category D.

As per my knowledge “SR based IV” field comes in contract from vendor master.

Is there any other way for “SR base IV” field  to over right for service contract in case vendor whose vendor master does not have a  SR base IV ticked in Purchasing view?

Also Please explain for Material contract why SR based IV is not by default mandatory term in contract?