When customizing an entry type, you both define how its entry editor should look, and what it takes for JabRef to consider an entry complete. You can both make changes to the existing entry types, and define new ones.
Selecting an entry type merely updates the dialog to show its set of required and optional fields, to give you a template for customization. You can then proceed to change both the definition and the name of the entry type before storing. If the name written in the Name text field is different from the selected item in the combo box, the Name field takes precedence when you click Store or Delete
You can delete a customized entry type by selecting it and clicking the Delete button; if the deleted definition redefined a standard type, the standard definition will be restored. You are not allowed to delete standard entry types.
Note that there is no undo when creating, changing and deleting entry type definitions.
When selecting one of the standard types from the combo box, the syntax for defining required and optional fields should become obvious; the field names are simply written in your preferred order, separated by semicolons. Either of the Required fields and Optional fields text boxes can be empty, if no fields fit into the category. The fields listed do not have to be standard fields known by JabRef.