Custom fields feature allows 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.
You canu use custom field to search invoices.
Custom fields columns are adding on the end of documents list but you can change their position. Just drag column caption and drop it in other place. Columns sequence will be changed and program will remember it after exit program.