Issue ID field refers to what?

Hi, forgive me if this has already been asked somewhere(I didn't see it in my short search.) I am curious what exactly the Issue ID field is referring to? I understand that it is a unique id attached to that issue but how was this ID derived? Is it an ambiguous ID given by the database or is it something unique to the actual issue like an ISBN or something? Anymore info would be appreciated, thanks! Michael

@pewsplosions: "ambiguous ID given by the database" is the correct answer.