Party A party to a hearing: a protestor; a protestee; a boat requesting redress; a boat or a competitor that may be penalized under rule 69.1; a race committee or an organizing authority in a hearing under rule 62.1(a).
When a boat, person, PC or OA become a party they have certain rights but also obligations. All these are specified in Part 5 of the RRS. For example only a party to a hearing may appeal a decision of a protest committee. If you've been affected by a redress hearing, because another boat has been placed above you in the scoring list, you cannot appeal that decision. You first have to request redress to the PC, in so doing become a party, and if that decision isn't what you want or think is correct, appeal.
A boat becomes a party either by protesting or being protested. The race committee or organizing authority become parties when a boat request redress, claiming that an improper action or omission by the RC or OA made their score significantly worse.
A party may bring witnesses to a hearing. It is not the PC who decides if a witness is to be heard, no, that is up to the party. They must however be available when called. That does not mean making a telephone call and arriving after 20 minutes. Available is next door, direct. A witness can become a party if the PC suspect a rule infringement by that boat. They must then first protest that boat, give her time to prepare and then continue with the hearing.
Parties have the right to be present when evidence is presented to the PC. They may question all witnesses and have a right to speak. The PC may direct the format, but may not exclude a party. If a party doesn't come to the hearing, provided all notices were correctly published, the PC may proceed without a party present. If later you can show that you could not attend because of a valid reason, the PC may reopen.
A written statement by a witness is usually not accepted, unless both parties agree. You can't question a piece of paper. If a party brings a tape of photographs to the hearing, it is up to the PC to determine if that evidence is accepted. In my experience most video's are not good enough, to far away, not on the line or otherwise inconclusive, to help in a hearing. But some of them do.
Only a party to a hearing may request a written report of the protest with facts found, conclusions and decision. Something you need if you want to appeal.