The button Recalculate amount is used if the payment plan shall be updated according to the order rows. This is useful when, for example, an addition has been made afterwards on the order rows and the order value is thereby higher than the payment plan amount. The partial invoice amounts will then be redistributed.
If the order rows are changed so that the order value is affected on already saved orders, a question appears asking if the payment plan shall be updated. If you say Yes, the Invoicing Plan tab opens and the Recalculate amount button is automatically updated. The check is made to avoid that the payment plan's total value will be incorrect.