There is a setting on Defaults \ Manage Suppliers Defaults - the Orders tab
To Select Products set to Preferred Supplier for Auto Order.
When this is set and a new Purchase Order is raised, and the button for Auto Order - Min /Max is used
The purchase order raised will only include products where their preferred supplier is the supplier that the order was raised against.
If the tick box is not ticked
When a new Purchase Order is raised, and the Auto Order - Min /Max used, the order will generate a line for any product that requires stock in the selected location. This will include products where the supplier on the order is a supplier for that product regardless of whether the supplier is the preferred supplier or not.
This is useful where a site may have multiple locations and use different supplier accounts for orders for each of the locations
i.e. the site wishes to use multiple supplier accounts for the same supplier so they can better manage their purchase debt.