On second thought, looks like the cert. is proof for "Date and method of acquisition of present Nationality", item 19. in Part B of the application. Under the 'enclosures for each applicant' section, they ask for copy of Passport / citizenship which seems to imply one or the other. But depending on the method of acquisition of present Nationality (like by birth, descent, natruralization etc.), which in our case is naturalization, both the documents become relevant. Passport shows I am a citizen and cert. shows proof of method used. In anycase, CGI Houston says they need it.