The folderlist form field type provides a drop down list of folders from a specfied directory. If the field has a saved value this is selected when the page is first loaded. If not, the default value (if any) is selected.
By default, the first item on the list is '- Do not use -' (which is translatable) and is given the value '-1' and this is followed by '- Use default -' (also translatable) given the value '0'.
Example XML field definition:
<field name="myfolder" type="folderlist" default="" label="Select a folder" directory="administrator" filter="" exclude="" stripext="" />