What is plagiarism exactly?
I suspect you can check the dictionary, if that is desired, while I will give a working definition/procedure from the standpoint of an academic instead.
Some plagiarism cases are easy (verbatim copying), some are difficult. Most investigations start by trying to be quite explicit concerning what was done, and why it should be considered "unethical" (somewhat of a mystical academic word). Moreover, local knowledge is often important. A law school will differ from a medical school, while maths and literature won't have the same standards either. Additionally, the particular standard to apply in a given case might depend on the nature of the offense. Sometimes a direct precedent is available, but more often than not there will be some novel aspects to the case. What is to be noted, though, is that at an early stage at least some standard (modifiable to some extent as things progress) is chosen
for the specific situation, so that the whole process is not endlessly side-tracked into the "meta-discussion" of generalities.
Since most examples from students on assignments (even in CS classes, where they can be quite inventive) tend to be rather perfunctory [and a teacher usually doesn't want to go thru the ordeal if it is not rather clear], I'll take "plagiarism" in a master's/doctoral thesis as my running example. It is quite untypical for an
entire thesis to be unoriginal. More frequent would be the copying of a smallish part of it, something like 3-5 pages (out of 100 in total) in a given section/chapter of the thesis -- unsurprisingly, this material is often that which is least well-understood by the student (it might be a side-bar that interlinks the subject of the thesis to something else, or perhaps some technical point that the advisor suggested should be addressed). The student will frequently reword (or even re-order) said "plagiarised" material, but in most cases it will still be apparent the student didn't really understand it, but just spliced it in (for instance, there will be no constructive discussion added). In many cases [depending on the subject matter], simply prefacing this by: "The section follows [...]" as a citation will suffice. Or the work in question can simply be excerpted -- this is, after all, a rather small percentage of thesis.
On the other hand, if such material is
not properly referenced, then this is indeed passing off someone's else work as one's own "original" work. One common excuse [and there are various famous persons who have copied parts of their thesis] is that it is a "small part" of the thesis, which I guess could be called "whitewashing" [rather than consider each part of the thesis, white or black, as an individual element for "plagiarism", just throw it all together and get a small percentage of grey]. Note that this is rather different from copyright [at least in the "damages" phase], where questions of percentages more often do arise. Usually those who are found to have plagiarised material in their thesis [or in other work] seek careers outside academia --- combined with a general lack of "half-way" penalties (you either revoke the awarded degree, or you, umm..., umm...), unless a case is particularly egregious and/or the offender ends up in politics, it is not likely to echo too far.
I could discuss how a plagiarism case is handled, but it's probably about how you would expect, with Commitees making subjective assessments, appeals to the Faculty Senate, or beyond, etc.
]. An additional point is that if the ICGA did use essentially a "copyright" standard, they would be more susceptible to lawsuits over their competence to adjudicate matters.