This tab allows you to customize the list of sites that will be available (shown) in the Sites drop down list on the main screen and the list under the Sites in the File menu on the main screen:

 

 

If you want the site to appear in the list, select it in the left list and click on the single Right Arrow between the lists to move it to the right.  If you want to remove the site from the list, select it in the right list and click on the single Left Arrow between the lists to move it to the right.  The Double Arrows between the lists will move the entire list to either side.