In an export declaration, supporting previous documents relating to the declaration are to be added as required by HMRC.
To enter previous document information at item level on a declaration, open the item and select the Details tab. In the Previous documents section, click New to create a new line for data entry.
Select the Category and Type, then enter the Reference.
To enter further previous documents, click the New button to create another line.
HMRC guidance for data element 2/1 can be found at this website.