Special Guest Stars

Do you ever watch television and in the credits you see Special Guest Star and then the person’s name? Then after all they list all of the special guest stars they list the people that are Guest Starring. My question is, when you’re an actor, when do you go from being a “guest star” to a “special guest star?” Is there like a check list worked into the SAG contracts? “Once the actor has met these qualifications they move from Guest Starring to Special Guest Star. Do actors aspire to make this leap? Is there a pay increase with this change in title, and if so, is it a percentage, or how is that worked out? I get that the actors that are deemed special are usually more well-known, but I can’t help but wonder, who makes the decision on when you can make the switch. Who decides that you are now mainstream enough to be classified as special? One person may be more well know to some than others. In fact, I know I’ve seen television shows where the special guest stars weren’t all that famous, but had been around (Hollywood, the producers, directors, or writers of the show) long enough to be deemed special. It’s things like this that keep me up at night (not really).