Hi,
We would recommend that you get familiar with the UI as this will help you spot how FInancials deals with the operations. Usually you can find the screenID for the module in the Swagger header of the endpoint.
The paymentLines of the customerPayment in the UI refers to the invoices that the payment should be applied to.
As customer can make a prepayment for example, the might have paid before you even have an invoice, in this case you could create a payment for them with an amount that has not been appluied to other documents.
If you have 1 or several paymentLines, you split up the totalAmount the customer payed on the lines as needed, each line representing one docunment to be paid.
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