You can divide your customers into groups that can be used as finance codes in a sales rule. The finance code groups you create can be used as a condition for where to book specific invoices in a sales rule.
Create a new finance code group.
Go to Sales > Customers > Finance code column, where you can create a finance code. Click on the field of the product and click on Edit.
Now, click on the green + New finance code and write the name. Click Save.
When you have created the groups, choose one for the customer, and it will be saved automatically.