All parameters are optional, except for client_id in a TransferAuthorization which is what differentiates it from a TransferRequest.
The idea is that most of them can be inferred.
For example from is normally inferred from the oauth token for Transfer and the currently logged in user for the TransferRequest/Authorization.
Amount could also be inferred such as in the Flattr where the amount is calculated by the system at the end of a period or a single usage credit in a usage virtual currency such as Twilio or SendGrid.
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