Document types typically contain all of the fields required to interpret an invoice from a particular country. However, sometimes you need to add extra information to the invoices during the verification process. For example, you can use a custom extraction field to add a reference to an external ERP system.
If you need to attach extra information to invoices, custom fields are an easy way to accomplish this. Instructions can be found in Help
Level of Complexity
Adding custom fields to a document type
Field format (compact)