This field is to sort a banana label in a catalog entry. For example if you like to sort all labels by farmnumber. This field will support you, so it is not an attribute of the bananalabel but of its position / ranking in a catalog entry. Please use it with care, you will not need it in 99% of the entries.
In the capter "Add a Bananalabels" it is explained in detail, how to get the labels in a nice order and how to use this field.