The assessed amount by the adjudicator is added to the Prices (the 'contract sum'), in full. Retention applies in relation to payments.
Under Main Option A or B, the amount assessed by the adjudicator is added to the Activity Schedule or Bill of Quantities and determines the payment amount, which is then subject to the retention provisions in your contract, which you say is 5%.
Under a Main Option C or D contract, payment is based on the Price for Work Done to Date, which is (actual and forecast) Defined Cost plus Fee, essentially what it 'actually' costs not the amount assessed by the adjudicator. This 'actual' amount is then subject to the retention provisions in your contract.
Under Main Option A or B, the amount assessed by the adjudicator is added to the Activity Schedule or Bill of Quantities and determines the payment amount, which is then subject to the retention provisions in your contract, which you say is 5%.
Under a Main Option C or D contract, payment is based on the Price for Work Done to Date, which is (actual and forecast) Defined Cost plus Fee, essentially what it 'actually' costs not the amount assessed by the adjudicator. This 'actual' amount is then subject to the retention provisions in your contract.