Glad I could help. By "official" they mean an attested copy. USEF Nepal is where Nepali certificates are attested so that universities in the US can be assured they are not dealing with fakes.

Look at the very last section on this page: http://usefnepal.org/eac-pages-22.html

Since you have your transcript with you, you will need to get a copy of the original from your university in nepal and have it attested at USEF.