When there are multiple purchase rows of data of for supplier (as shown below), the preferred rule indicates which purchase data is preferred.
In the example above, there is 1 case with 1 unit, and 1 case with 6 units from the same supplier. This makes it possible to order the product by 1 unit or by 1 case of 6 as well.
When you double click on the row, the box 'Preferred supplier row' indicates what the preference is when purchasing.
If there is only 1 purchase data from a supplier, then that is always the preference, because there is no other purchasing information that could be.
In that case it is not possible to uncheck the preference line.