The due days are added to the invoice date to calculate the due date. By default, the Invoice date will be the date on which you create an invoice.The due days includes the following,

1. Due on Receipt-Payment is due immediately.
2. Net 15-Payment is due in 15 days.
3. Net 30-Payment is due in 30 days.
4. Net 60-payment is due in 60 days.
Note:
The term 'Net' indicates that when the payment is due. You can also create your own terms using Add New option.