A billogram is a collection of related invoices, and the base information used to create them. To invoice a customer, you create a billogram with the items to be invoiced for, assign the customer to it, and attest it by performing the send command. This generates an initial invoice to the customer. If circumstances later change and you need to e.g. credit part of the invoice, the credit invoice for this will be added to the same billogram, keeping related information together. The billogram object also lets you change the items invoiced for after sending the invoice, in this case the Billogram service will intelligently create additional regular invoices and credit invoices as required to ensure the customer pays the correct amount.

In the Billogram system, invoices are represented as sub-objects of billogram objects, and invoices are immutable. Changes to the billogram object will rather generate new invoices.

A billogram object also contains a complete history of all activity relating to it, such as it being created, invoices sent, payments received, reminders sent, and so on.