A settlement is always tied to exactly one consignor and one sale. It gathers every lot that consignor sold in that sale, totals the proceeds, subtracts the fees, and produces the amount they get paid. The consignor's record also drives a few settlement behaviors — their running balance is applied here, and their beef checkoff exemption flag decides whether that fee is charged.
You can't change the consignor on an existing settlement. If a settlement was created for the wrong seller, void it and create a new one.