immigrant visa/221g/F2-B Retention Letter

[athena03]

This is abt my sister she just had her U.S Visa Interview today however when asked about our petitioner's status she was unsure and answered that our petitioner was a U.S Citizen when she is actually a Permanent Resident Card holder. They actually requested an F2-B Retention Letter because of her mistake in informing them about our petitioner being a U.S. Citizen But I have the same documents as what my sister had, I was given a U.S. Visa but I do know and informed that our petitioner (our mother) has Permanent Resident Card. She was informed that since she stated that our mother was a U.S Citizen she is being required an Approve F2-B Retention Letter, but that was a mistake & as I understood correctly that document is being required if the petitioner is already a U.S. Citizen. Just to fully understand I would like to know what is the F2-B Retention Letter? what is stated in that document? Both me & my sister are Principal Applicants but I was already given a U.S Visa last October along w/ my 3 kids.