Documents can either be Standalone documents or Referring documents. A Standalone document doesn't refer to anything. A Referring document refers to a CCI number or to an object existing in SFC2021.
For referential documents, when a CCI link is available it means that the document can be linked to a CCI number related to the type of document. CCI numbers are allocated before the related object is encoded in SFC2021. When a Programme (PRG) link is available it means that the document can be linked to a Programme already encoded in SFC2021.