Do you want to see campaigns or projects separately on your invoice? Then use cost centres via the billingId field.
You can include your own billingId with every order. You will see this on the monthly invoice as a separate specification.
Suppose you want to track all costs for your "Kerst 2025" (Christmas 2025) campaign separately, then use billingId: "Kerst 2025" when creating those orders. On your invoice, these costs will then be neatly grouped under that name.
Only use the billingId for clear categories (such as campaigns or departments). Do not use it for unique or random IDs per order. This makes the invoice unnecessarily confusing.
More technical explanation and examples can be found in the API documentation.