GOV.UK
First-tier and Upper Tribunals: Introduction: Definitions
An appellant is the person who starts proceedings, whether by appealing to the tribunal or making an application to the tribunal. This definition can include HMRC if we bring proceedings, for example by making an application for permission to issue an information notice.
A respondent means HMRC, where the appellant is not HMRC. If HMRC brings the proceedings the respondent is the person against whom proceedings are brought or to whom proceedings relate.
A party to the proceedings is a person who is an appellant or respondent in proceedings before the tribunal