See ISPE Guidelines 4.26.

Where the reference document relates to the background art, it may be in the application as originally filed or introduced at a later date (see GL/PCT‑EPO F‑II, 4.3, and GL/PCT‑EPO H‑II, 2.2.5). Incorporation of essential matter or essential features at a later date is, however, subject to the restrictions set out in GL/PCT‑EPO H‑II, 2.2.1. It may be that the examiner has requested the applicant to furnish the document referred to, in order to be able to carry out a meaningful search (see ISPE Guidelines 15.37).