Upon starting the payments which properties are required?
The new properties marked with 3DS badge are highly recommended. (Read: error won't be thrown, when sent without these properties). These content of these fields are optional. Payments undergo Transaction Risk Analysis on the card issuer side based on these data as well, deciding if the payment should be challenged or not.
Are TraceId required for earlier initialized token payments after 2021.02.01?
First yes, TraceIds are not required for earlier initialized token payments without 3DS for an INTERIM period. After this period expires (potentially 2021 Q3) TraceIds will be required for Token Payments.
We've received TraceId in the following format:
The TraceId is generated by the card issuer, and can be anything up to 100 characters. Whitespaces and 0s should also be left intact.
Can we skip the additional data in subsequent payments required for 3DS authentication in the first Token Payment?
Wrong! The difference between the initial and subsequent token payment is that 3DS authentication is only required for the first, initial payment. More info can be found here
Do we have to generate the "TraceId" in the first Payment/Start, or is it generated by Barion?
TraceId is generated by the Card Issuer, so it is not required in the initial Payment6Start. You have to store the TraceId, recived int ghe GetPaymentState, and send in all subsequent payments. You can read more here. The exceptions are the subsequent payments of an already initialized token payments without 3DS. More info can be found here
When the amount of the first payment is different than the rest, I have to use MechantInitiatedPayment and not Recurring. Right?
Yes, RecurringPayment can only be used, when the amounts of all the subsequent payments are the same. If only 1 is different (eg. you give discount/free for the first month), only MechantInitiatedPayment can be used.