THE ADMINISTRATION'S policy is clear: that certain limited categories of refugees from Cuba will be admitted to the United States. The reality is infinitely more compelling and anguished: that people of every possible category, or none at all, are arriving by the thousands and tens of thousands in a gigantic spontaneous exodus from Fidel Castro's island. Mr. Castro is now deliberately throwing into this stream the criminal, the sick, the insane -- people whom no orderly immigration program could possibly accept.

Mr. Castro knows as well as anyone else that the exodus is the most devastating kind of judgment, by the best informed of judges, on life under his style of socialism, 21 years after the revolution. He is urgently anxious to bait the United States into a public quarrel over who's fit to enter Florida -- a quarrel calculated to divert attention from one country's failures to a mean-spirited legalism on the part of the other.

What's the United States to do? The wise and decent choice is to err on the side of generosity toward the refugees.

That choice -- let us acknowledge it -- breaks the customary rules and standards developed through long and sometimes painful experience with immigration. Ideally, it would be vastly better for the United States to set up a screening system inside Cuba, enabling American officials to find out exactly whom, in each case, they are dealing with. But the Cuban authorities are not cooperating. American diplomats in Havana tried to organize just that kind of an orderly application procedure in the U.S. mission there. But men armed with clubs and chains, evidently Cuban security police, attacked the people standing in line, who fled into the mission. Several hundred of them are still there, with the building encircled by Cuban police.

There are going to be a lot of questionable cases, and worse, among those thousands of people coming ashore in Florida. This woman says that she's that woman's aunt. Since nobody has any documents, the rule about family ties won't be simple to enforce. What about the people who, according to Havana, are common criminals, but who say that they were jailed for political offenses? What about the upstanding young man with a wife, a baby and an active case of tuberculosis? There's a good deal of TB coming ashore in those boats. Then there's the poor soul who is profoundly psychotic, pushed onto a boat by the Cuban government. Who's going to deliver him back to Cuba, and how?

While Mr. Castro is sending along some people that neither the United States nor any other country would willingly admit, he is apparently also holding back many of those most eligible -- the relatives of people already here. Among Cuban-Americans, there is now rising anxiety over the inability to get these people out. Perhaps a million Cubans might meet the current definition of a relative.

As everyone knows, it's not only Cubans who are coming. What about Haitians? Not many Americans would favor letting in Hispanic refugees from a left-wing authoritarian regime, to gain a political point, while shutting out black refugees from a conventional right-wing authoritarian regime. Haiti lives in greater poverty than Cuba, and its government is totally destitute of any regard for human rights. If the Haitians can land, there are also the Jamaicans. Their government is genuinely democratic but their economy is hardly more prosperous than Cuba's.

No one can say how many Cubans are likely to come into this country, even under the restrictive limits that the Carter administration has tried to set. Since, as we suggest, those limits will be largely unenforceable in practice, the numbers will be large. With a more open policy, it would be necessary to expect hundreds of thousands.

It's going to be far more disorderly than anyone likes, this process of admitting some unknown but large number of new Americans from Cuba and other Caribbean countries. But since we have been speaking of realities, let us mention one more. These refugees are, in the vast majority, decent and diligent people. They have come here in the hope of working toward better lives for themselves and their children. By better lives, they mean more to eat, more education, more opportunities and personal freedom. Their hopes and aspiration will be a valuable contribution to this country. The right policy is the one that resolves the hard choices in favor of letting refugees in.