When you insert balloons, you must hover over the entity to highlight the entity and to attach the leader. The leader does not appear until you hover over the entity. This way, the leader and highlighted entities do not hinder your view of the model or drawing view.
Video: Balloons with Auto Leaders
A balloon containing an item number attaches to the part. If you specified the text to be Item Number, the number in the balloon corresponds to the item number in the bill of materials.