To get this labels sorted by farmnumber, you can add the farmnumber in the SORTFIELD. All existing labels in this catalog entry have got the farmnumber in the SORTFIELD, so it should not to much work to add this number if you add a new label?

But you are right, the SORTFIELD is not needed to find a label. In this case you use the "Search Text". It is only to have the labels in the catalog entry in a nice order.