Copy of your whole passport with initial entry visa stamp on the passport ( including blank page).
I guess in your case, the possible reasons for this ref. is :1. The principle petitioner came to US on Sept. 1998, and you joined to him/her late; or 2. Your H1 or F1 was approved on Sept. 1998, but you came on 2000. That is why USCIS needs you to give the evidence during this period.
Good Lock.