Assigning an additional code list to a custom extension field Complete these steps to assign an additional code list to a custom extension field. Open Analysis Dimensions (AND) and select, or create, your analysis dimension. Click OK. Select Actions > Extension Fields and select the custom extension field you want to attach the additional code list to, and click Amend. The Extension Fields Setup form is displayed. Amend the Reference Code List field to include your new additional code list. The new codes are stored in the Default Value field.