Custom fields feature let you add some additional fields to invoice. For example you need to show VIN (Vehicle identification number) on Invoice. So first select any custom text field and enable "Use this field" option. Enter field name (eg. VIN) and save settings.
Now during create new Invoice new field will be available for you so you can use it to type in VIN.
Additionaly on documents list VIN column will be visible.
Use 'Show on invoice' option to show this field on Invoice.Tips: