In the UK, an AG's Reference is used by the Government, usually to appeal an acquittal on a point of law - but nonetheless double jeopardy generally means that even if it is found that a defendant was incorrectly acquitted on a legal mistake, the Court of Appeal/HoL's decision only affects subsequent cases, not the incorrectly acquitted defendant.

In other Commonwealth jurisdictions, the same true to the best of my knowledge; in the US, the same title may be given to an opinion from a state or federal AG - but I'm not sure about that last one.

One of the primary duties of the attorney-general is to advise on the law. Thus, AG's write opinion letters informing public officials and the public what a specific law requires. A reference is just a cite to one of these opinion letters.