Also known as special guest referee is any match in which the usual referee is replaced with a "guest" filling in as the official. Celebrities (such as Muhammad Ali in the main event of WrestleMania), managers and other wrestlers can "guest" as the special referee. In some cases, a special referee is put into a match which is already a different match type or stipulation (for example: Hell in a Cell with a Special Referee). The special referee will often be biased towards or against one of the competitors or will be assigned as the Special Referee to ensure the match is called down the line. In September 1999, in the WWE, after all the referees got sick of continuously being attacked by wrestlers, they went on strike (kayfabe), leading to other WWE workers (most notably Harvey Whippleman and Tom Prichard, along with a non-striking Jim Korderas) becoming "scab" referees until the night after Unforgiven, where Vince McMahon gave the regular referees more authority in matches (along with fining Triple H for striking one that same night).
Special outside referee
Also known as Special Enforcer or Special Guest Enforcer is same as the Special Referee but the guest referee stays on the outside of the ring enforcing what the normal referee doesn't see. These guests are sometimes known as "enforcers", the most famous of which was Mike Tyson, who served as the Special Guest Enforcer for the WWF title match between Steve Austin and Shawn Michaels at WrestleMania XIV, and Chuck Norris who served as Special Guest Enforcer at Survivor Series 1994 in a match between The Undertaker and Yokozuna.
Special Enforcers can become regular referees if the original inside referee becomes (kayfabe) permanently incapacitated. Otherwise, though, the enforcer generally has no decision-making power, and is really put in the match to physically force wrestlers to obey the rules or physically remove interfering wrestlers from ringside.