Field descriptions:

fb_type_id: See Predefined values
created_time: time the payout has been created by the payment provider
amount: transferred amount to the bank account
amount_donations: sum of the amount of the assigned fb_donations
amount_donations_fee: sum of the payment provider fees of the assigend fb_donations (negative number)
amount_reversed_donations: sum of the amount of the recognized reversed fb_donations (negative number)
amount_reversed_donations_fee: sum of the payment provider fees of the recognized reversed fb_donations (can be negative (e.g. dispute fee) or positive (e.g. refunded fee))
amount_unknown: sum of the amount of the unknown transactions found in this payout (not initiated by FundraisingBox)
currency: See Predefined values

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Retrieve connected fb_donations
If you want to retrieve the connected donations to a payout, you can use the donations-API with criteria fb_payout_id, for example:
GET /donations.xml?criteria[fb_payout_id]={id}