The certain type of hard drinker is someone who drinks a lot but is not as far gone as the alcoholic.
Here's the passage in the Big Book:
Then we have a certain type of hard drinker. He may have the habit badly enough to gradually impair him physically and mentally. It may cause him to die a few years before his time. If a sufficiently strong reason—ill health, falling in love, change of environment, or the warning of a doctor—becomes operative, this man can also stop or moderate, although he may find it difficult and troublesome and may even need medical attention.
Imagine three people, Albert, Bobby, and Charlie. All three start drinking at 15, overshoot every time, and have bad consequences. They are all due to die at 65 as a direct result of drinking.
Albert, at 50, decides enough is enough and stops (or moderates). He lives to a ripe old age. No AA. He is a certain type of hard drinker.
Bobby, at 50, decides enough is enough but cannot stop and cannot moderate. He goes to AA. He lives to a ripe old age. He is an alcoholic of our type.
Charlie, at 50, never decides enough is enough, continues to drink, and dies of ruptured oesophageal varices at the age of 65.
What is Charlie? An alcoholic? Or a certain type of hard drinker?
His drinking is the same as Albert's and Bobby's, but his thinking is so warped that, even though alcohol is killing him, and does indeed kill him, he never even conceives of stopping.
He is clearly worse than Bobby. If Bobby is an alcoholic, Charlie is definitely an alcoholic. Alcoholism is not the Goldilocks of drinking problems, not so mild as can be overcome by willpower, but not so severe as to warp the mind into never seeking a solution. Someone who never even seeks a solution to the obvious problem has a more severe case, not a milder case.
The test for the certain type of hard drinker is not whether the individual wanted to stop or tried to stop but whether, given the consequences they endured, they should have tried to stop.
Alcoholics thus fall into two categories:
(1) Heavy drinkers who have it bad, but are still sharp enough to realise, so try to stop yet fail.
(2) Heavy drinkers who have it bad, but are too blunted, weakened, or craven to even try to stop.
Do 'certain types of hard drinker' ever come to AA?
Imagine 26 people, Andrew, Bertie, Colin, David, etc., through to Zechariah.
All drink five days a week from the age of 20 to 30.
On two days a week, bad things happen when they drink, and, on balance, it was a very bad idea.
That means, over 10 years, they have about 1000 bad experiences.
These experiences start to pile up, and, once the pile reaches a tipping point, the individual decides to stop and moderate.
First Andrew stops, after a few bad experiences, then Bertie, then Colin, etc., all the way through to Yannis. The only person who does not try to stop (despite identical bad experiences) is Zechariah.
So Andrew through to Yannis attempt to stop and moderate, one by one, and one by one they succeed in either stopping or moderating.
All except Yannis. By the time Yannis gets to 30, he has been trying to stop or moderate for ten years, and has failed. The rest have succeeded.
So Yannis comes to AA. Could he be a hard drinker? No. If he were, he would not have come to AA. Andrew through to Xavier are certain types of hard drinker: they can stop or moderate, and so they did. Since they did, they do not seek out AA.
We're left with Zechariah. Well, Zechariah, who obviously has it the worst of the lot. He's the most warped. He has exactly the same experiences as the others, but his will is so damaged he cannot even muster the desire or willingness to stop.
Most Zechariahs just die of alcoholism.
When they come to AA, though, they are definitionally not a certain type of hard drinker. They have a sufficiently strong reason to stop, which is why they are at AA, and they have had ample opportunity over the years or decades to moderate or stop, and clearly haven't, so they are not a certain type of hard drinker. Zechariahs are alcoholics of our type.