You can store additional metadata with Klarna that will be returned whenever an order is read from Klarna. The data take two forms: two reference fields to be used to store your internal order reference, and general metadata fields on the order and on each order line that can be used to store arbitrary data.
The reference fields are
merchant_reference2 used to store the interal reference to the order.
The general metadata fields are
merchant_data on the order and
merchant_data on each order line.