What's"All documents establishing your eligibility for Lawful PR

FB 485 interview is coming soon. Although most documents have been submitted with application of 130/485, the interview notice still contains a check list, and on which there are a few items I don't know:

1.All documents establishing your eligibility for Lawful PR status. Beside forms and supporting materials, what would be the typical documents for this?

2. Evidence of sponsor's US citizenship? Would a US birth certificate work? The sponsor does not have a possport yet.

Thank you very much for any help.