legal presence

The Secretary of State requires proof that your client is either a U.S. Citizen or an immigrant who is lawfully present in the U.S.  

You must provide at least one document showing legal presence in the United States

 If your client is a U.S. Citizen,  then usually a birth certificate is the best place to start.  If your client is not a U.S. citizen,  then your client will have to produce papers showing that he or she is here legally.   The proof is different depending on whether your client has  an I-90 form, known as a "green card".