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, the lowest price per piece differs. This gives you the option to order the product with a different price per piece.
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.