The List box control type allows you to choose from a pre-defined list that you have typed in or from the entries in an already existing database field. When you attempt to enter data in the box in your document, it will only accept data that is in the list you typed or in the field that you selected, otherwise it will display the first entry in the list if you try to enter invalid data. The List box is useful if you want to restrict the data displayed to a pre-set selection. If you sometimes want the option to enter data that is not in a pre-set list, then it is better to use the Combo box i.e. combination box, control type.
Example
When inserting a formula field you can set the Control Type at List box.
On the next page of the "Insert Field Wizard", select Use Choices in the Control Source box, and then type your list choices (one per line) in the Choices box.
Alternatively, if you click on Lookup Choices you can use a pre-set list contained in a database field.
See: