This field is accessible when the sort field name is present several times amongst the first fifteen columns comprising the presentation. It is used in this case to specify the table containing the sort field.
For example, this field is useful in the case of the "Country" field. The user can define a selection support displaying the Country field of the BP and the Country field of the Address at the same time. These two fields share an identical code. If the user wishes to sort by country, it is indispensable to specify which of the two countries should be used. Should it be the country from the BP table or the Country from the Address table?