This dialog is displayed when you edit a User-defined field (UDF) in Tools / Options.... Use it to define additional fields that can be associated with Issues, and to redefine existing UDFs. In addition to specifying the type of data stored by the field, you can specify a default value and a list of options for the user to select from.
This dialog is only available to Admin users.
Click What's This? Help in the dialog box for field definitions.