Redeeming DOC before the settlement is charged with a fee, you can queue your redeem for next settlement to avoid that.
It is “free” because what is redeemed are the “FREE DOCs”. You can check this rate when you exchange DOC, it is below DOC Fee.
This is quite technical into the model: the DOCs that are not free are in “bucket2”, and are the counterparty of current BTC2X Operations.
Maybe DOC REDEMPTION RATE will generate less confusion. We will consider this change in the next front-end version.