Most field item property pages are easy to understand but the List and Combo options are a little more complicated. 

Adding items to list/combo boxes

Open the field Properties dialog (see Adding form fields) and select the Options tab.

Enter the item's label in the Label: text box. This will be displayed to the user as part of the field's drop-down list. 

Enter any text or number in the Export value: text box. This does not get displayed but is stored inside the form to represent the value chosen if the user picks this item.

Press Add and the item label will be added to the Items list below.

Setting a default item

Click on an item in the Items list and then press Set

Moving items within the list

To change the order of items in the list, select a label in the Items list then press Up or Down to move it by one position. If you just want the entire list to appear sorted alphabetically, put a tick in the Sort items check box. The list will then be sorted when it pops up in front of a user.