Typical use case

You have collected all the data of the transaction, including the identifier of the Buyer Bank, and you want to initiate a MyBank Transaction.

Request body attributes
All attributes in the request body are mandatory except:

durationMillis (if not specified, a value corresponding to '15 minutes' will be set by default)
merchantTaxNumberClassification (only needed if the optional feature "SCT for fiscal benefits" is requested)
merchantTaxNumberID (only needed if the optional feature "SCT for fiscal benefits" is requested)

About initiatingPartyReturnURL

Your initiatingPartyReturnURL must be a unique and per-transaction URL: for example, https://www.mystore.com/transactionHasBeenHandledByPayer?myid=123456 would be a correct value, assuming that you will identify this MyBank Transaction by "123456".
After authorising or cancelling the Transaction, the Payer may ask to be redirected to this URL. In other terms, the fact that this URL is requested means that the final status of the Transaction is available, and ready to be retrieved by invoking paymentResultSCT01.

When initiatingPartyReturnURL is requested you should typically:

Retrieve the final status of the MyBank Transaction by invoking paymentResultSCT01.
Serve a confirmation page to the Payer, stating that the payment has been successful / unsuccessful.

IMPORTANT:

Even though, in a typical scenario where the payment flow is initiated from a browser, the initiatingPartyReturnURL will be requested in the same browser, this URL must be designed so that it can be handled without relying on any "session". (In case this URL further redirects to a merchant's return URL, the Payee PSP must ensure that the merchant's return URL does not rely on any session either).
Sample scenario where this would be relevant: payment flow initiated from a non-default browser (the initiatingPartyReturnURL may be requested in the default browser instead).
In case the payment flow is initiated from an App, it is recommended that the App is registered to handle the initiatingPartyReturnURL (if not, the initiatingPartyReturnURL will be requested in the browser: some mechanism will be then required to update the content of the App).
Please be reminded that the initiatingPartyReturnURL must not be the only trigger for the retrieval of the status of a transaction: the Payer might close the browser immediately after authorising a transaction on the bank's Home Banking, before the request to initiatingPartyReturnURL is launched. Payee PSPs MUST implement an appropriate mechanism to retrieve the status of transactions for which no return redirection occurs, and must ensure that their merchants can complete their business processes accordingly.

Typical use of the response

You will store the transactionID for later use: when your initiatingPartyReturnURL is requested, or when the Transaction expires, you will use transactionID to invoke paymentResultSCT01.
You will redirect the Payer to the validationServiceURL.