To answer your first question, Cannabis Use Disorder specifically refers to a pattern of use that leads to health problems, disability, and failure to meet important responsibilities at work/school/home. So it's not defined by use but rather by consequences of use.
Second, there are a number of large cohort studies looking at this question. Here's a recent one:
http://www.ncbi.nlm.nih.gov/pubmed/26006253
Psychosocial sequelae of cannabis use and implications for policy: findings from the Christchurch Health and Development Study.
It followed a group of 1200+ kids in New Zealand for all kinds of stuff, but in this particular paper they looked at cannabis use.
Their conclusion:
In general, the findings of the CHDS suggest that individuals who use cannabis regularly, or who begin using cannabis at earlier ages, are at increased risk of a range of adverse outcomes, including: lower levels of educational attainment; welfare dependence and unemployment; using other, more dangerous illicit drugs; and psychotic symptomatology. It should also be noted, however, that there is a substantial proportion of regular adult users who do not experience harmful consequences as a result of cannabis use.
In conclusion, it seems that some adults can use occasionally, or even regularly, without a problem on one of the outcomes they looked at. Those with a personal or family history of addiction or a psychiatric disorder are probably at higher risk for a problem.
But, it comes back the same problem mentioned before: Every study can only answer the narrow questions they have considered, How does cannabis impact the risk for vascular disease, or dementia? Perhaps it helps, perhaps it hurts.This study does not answer that question.
