I'm a licensed mental health therapist who has had experience performing substance abuse and chemical dependence evaluations. There are two types of substance use diagnoses:
1. Substance Abuse (Alcohol Abuse)
2. Substance Dependence (Alcohol Dependence)
These can be applied not only to alcohol, but to other substances as well. Honestly, to be diagonsed as someone with "alcohol abuse", it doesn't take much. If you're out binge drinking 1-2 times a week, you could very well be diagnosed as being someone that abuses alcohol. The average college kid who drinks and parties on a regular basis would fit this diagnosis. I could probably diagnose myself as someone with "alcohol abuse", depending on when an evaluation was done on me.
When abuse turns into dependence, this is when it becomes more worrisome. As I saw in previous posts, someone explained what a functioning alcoholic looks like. They described that person as someone who is able to go through life, funciton normally, and form relationships, etc. despite that individual's alcohol use. I have counseled many people that fit that label.
Alcohol or substance dependence can be defined as someone who regularly relies on regularly using alcohol or drug/substance for an extended period of time. That period being 6 months or more. You also have to take other factors into consideration. Those factors being legal issues, relationship issues, career or job issues, physical health, mental health, emotional health, withdrawl issues, etc. If even some of those are what we as therapists would define as being inhibited or problematic, then that individual could be diagnosed as being someone with alcohol or substance use dependence.
For that person to not be labeled as someone with alcohol or substance use dependence, they have to have at least 6 months of complete sobriety. In a substance use evaluation, you can even diagnose someone that would be classified as "alcohol dependence" if it was something in their past. You would just diagnose it and classify it as something that could or was diagnosed in the past. If you want to get more technical, sometimes it can be diagnosed as "alcohol use disorder". That's more broad as it doesn't classify abuse or dependence as I had mentioned before.
It's really up to the clinician to diagnose what they think is appropriate, but following the guidelines in the DSM-V, doing a thorough evaluation consisting of past history (physical, mental, substance use, education, family, etc.), and using evidenced based assessments like the "SASSI" and other alcohol assements can also add to the ability to diagnose and allow for more evidence/reliablity on why you're diagnosing an individual with a certain disorder. Sometimes diagnosing someone can be subjective, depending on the individual. But, if you use what you know about the patient, proper assessment tools, and do a thorough evaluation, it's much easier to obtain a proper diagnosis.
I hope this helps and answers all your questions!