Day of the month: Invoices using this term are always paid on selected days during each month. For example, you may always pay your invoices on the 1st and 15th of the month.
Number of days: Invoices using this term are always paid a certain number of days after the invoice date. When you select this type, you can enter the number of days until payment is due, and the number of days you will receive a discount (optional).

Invoice date: the days until due is added to the invoice date to calculate the due date
Invoice received date: the days until due is added to the invoice’s date received to calculate the due date
Most recent expense date: the days until due is added to the latest expense date on the invoice’s lines. For example, if the invoice is matched to six receipts, the date on which the latest of the receipts was received is used.