When Affirmed faces Seattle Slew at Belmont Park today, the outcome of their confrontation may be determined as much by racing tactics as by the raw ability of the horses.

The man who can dictate the tactics in the $300,000 Jockey Club Gold Cup is Laz Barrera, the trainer of Affirmed, but his position is not as enviable as it sounds. He has the choice of two strategies, either of which is likely to prove disastrous.

Should Affirmed challenge Slew as soon as they come out of the gate, running the risk that they will battle each other into defeat and set up the race for their stretch-running rival, Exceller.

Or should Affirmed permit Slew to go to the front, running the risk that he will set a slow pace and steal off to an insurmountable early lead?

That is exactly what happened when the two Triple Crown winners met in the Marlboro Cup a month ago. Jockey Steven Cauthen let Slew take a quick three-length lead and Affirmed could never get close to him. Affirmed was thus deprived of the chance to use his greatest asset: the tenacity in head-to-head battles that he displayed in his memorable victories over Alydar this spring.

Barrera believes there is no horse that can outduel Affirmed in head-to-head combat, and he said, "I'd like to send Affirmed right after Seattle Slew. But I can't - not with Exceller in the race."

In the Triple Crown, Affirmed and Alydar could race stride for stride around the track because their jockeys knew they had no one to fear but each other. Exceller, however, is a horse to be feared. Although casual racing fans may not know his name, he comes into the Gold Cup with achievements that are just as distingushed as his more illustrious rivals.

Excellar won major races in England, France and Canada before he came to California this year. He proved himself the best grass runner in America and then showed he was equally adapt on dirt, beating the West Coast horses at their own game.

Exceller lost to Seattle Slew by four lengths in the Woodward Stakes two weeks ago, but he was at every disadvantage that day. He was making his first start in two months; the 1 1/4-mile distance was shorter than he likes, and because there was no speed in the race, he had to alter his usual stretch-running style and chase Slew early.

But at today's 1 1/2-mile distance, Exceller will be very formidable, and Barrera knows it.

"I can't leave myself open to Exceller coming from behind," he said. "You just can't go a mile and a half a head and head with another horse when there's someone else in the field who can come from behind."

Because he finds his tactical alternatives unacceptable, Barrera has rejected them both and settled on a third strategy.

He has entered a stablemate, Life's Hope, in the Gold Cup (along with another horse, Americanized, which is unlikely to start.) Life's Hope is a stakes-quality runner with some speed, and Barrera would like him to do the dirty work of pressing Seattle Slew early while Affirmed stalks them both.

The practice of using a pacemaker, a "rabbit," is an old and often effective racing strategy. Barrera remembers when he was training the favorite for a race in Mexico years ago, and a rival trainer sent a five-horse entry against him. The stablemares ran at Barrera's horse in relays, and finally raced him into defeat. "It was a lesson I never forgot," Barrera said.

But the strategy may not work for Barrera this time. Life's Hope may not have enough speed to stay with Seattle Slew. Even if he does, jockey Angel Cordero Jr. is smart enough, and Slew tractable enough, to ignore the rabbit's presence. The only way Slew will be forced to run hard early is if Affirmed makes him do it. For Barrera, that is the Catch 22 of the Jockey Club Gold Cup.