Invoice Groups and Installments
You can enable the Invoice Groups feature if the Installments feature is enabled, but an invoice with an installment term cannot be grouped. If you select an installment term on a sales order or invoice for a customer that has invoice grouping enabled, the transaction is not saved and an error message appears. To save a transaction with an installment term, you must clear the For Invoice Grouping box. If the term changes from installments to a different term, the Group Invoices box is checked. and the invoice is eligible for grouping.
If an invoice is marked for grouping, the Term field is dimmed.
For more information about the Installments feature, see Creating Installments