A jury duty exemption letter is a response to a jury summons that states a given reason why the potential juror is exempted from serving jury duty. Each state has specific exemptions pertaining to jury duty, and most of them require a letter requesting the exemption.
Personal situations can result in exemption from jury duty if addressed via an exemption letter. Some states list specific exemptions on the jury summons mailer that can be marked and sent back. Normally, a response to the exemption letter is needed before a potential juror can be dismissed from appearing on the date summoned.