How is the union flag used at the assignment page?

At the assignment page, you can maintain the Bargaining unit, Collective bargaining agreement and the Union check box. Although the Bargaining unit and the Collective bargaining agreement are linked, the Union check box is user maintained i.e. this check box needs to be selected by the user. Although a autocomplete rule can be created to maintain the Union flag, how is the Union flag used?

Are there any implications if the Union flag is not selected but the Bargaining unit and Collective bargaining agreement have been populated?