A payment term is a condition or guideline under which you can make or receive a payment. You must include a payment term on the invoice so your customers know that they must pay outstanding charges within a certain time. You can create multiple payment terms, but only one payment term can be made default at a time. To make a payment term, default, the "IsDefault” valuein the payload must be set to True.
If there is only one payment term, Payment Term-1, in the org and the value IsDefault is set to True in the payload, it is automatically set as a default payment term.
If another Payment Term-2, is marked as default, Payment Term-1 ceases to be the default payment term.
Similarly, if Payment Term-3 is marked as default, Payment Term-2 ceases to be the default payment term, and so on.
To make Payment Term-1 a default payment term again, you must call the billing services endpoint.