On this tab, you can see the table's default column (field) names. In these fields you can edit only the Field section. You can add your custom field using the New attribute () icon. You can also Delete () fields, move them Up () and Down () or add a new fields category using the New category () icon.
Please note: When creating a new empty field in a system table, it is highly recommended that you always allow empty values in the newly created field, so that the system procedures and existing dialogs keep working.
More information about system tables can be found in Developer's Guide -> Development -> System tables and custom fields.