When you create a ticket, it is attached to a customer. That customer will then receive all updates and communication as work is being done on the ticket. Once that ticket is complete and the work is ready for payment, an invoice is then created from the ticket so that the two are linked together.
By default, the invoice will be created for the same customer as was listed on the ticket. However, clicking Change in the Bill To Customer area will allow you to modify this.
After clicking Change, you will be prompted to select the new customer that will be billed for the invoice. Enter the customer's name and click "Attach It" to save the change.
Once the new billing customer has been selected, a new section will be added to the invoice to show the original customer that belongs to the ticket allowing for easy access to information all from the same page.