An overflow leader or overflow trailer is a positioned subform that can be assigned to any repeating subform. Typically, you create the subform that will become overflow leader or trailer first. Then you assign them to a repeating subform and give them overflow properties.
If the overflow leader is also treated as a bookend leader, it is placed in the Hierarchy palette just above the subform. Also, if the overflow trailer is also treated as a bookend trailer, it is placed in the Hierarchy palette just below the subform.
Look at the Subform tab in the Object palette. Ensure that the Content list displays Positioned. In the Object palette, click the Subform tab and make sure that Positioned is selected in the Content list.
Look at the Binding Tab in the Object palette. In the Object palette, click Binding tab and select Repeat Subform For Each Data Item to have the leaders and trailers appear for more than one overflow. Bookend leader and trailers are not counted against this number.
4.
If you want to specify an overflow leader, in the Overflow Leader list, select the subform that you want to use as the overflow leader subform for the current repeating subform. Alternatively, select New from the Overflow Leader list to create and assign a new overflow leader subform.
5.
If you want to specify an overflow trailer, in the Overflow Trailer list, select the subform that you want to use as the overflow trailer subform for the current repeating subform. Alternatively, select New from the Overflow Trailer list to create and assign a new overflow trailer subform.
Note:
You do not need to have both an overflow leader and an overflow trailer. Whether you have one, both, or none is determined entirely by the requirements of your form.
See also 

To create and assign overflow leaders and trailers