That section is awkwardly worded. What it refers to is that the Attorney General may intervene in cases at first instance or in appeal, as you point out.
There are two types of cases that could fall within the scope of section 14. There are cases we conduct the prosecution of. It would be very unlikely that the Attorney General would intervene in a case that we're conducting the prosecution of--and that hasn't happened. And there are cases in which the provincial attorneys general are the prosecutors, and there's a section of the Criminal Code that's been challenged as being contrary to the charter, or there is an investigative technique that is an issue, for example protection of informant privilege. In those cases a decision has to be taken: should we intervene, as the DPP or the Attorney General, to in effect put our point of view before the court if we have something useful to add to the debate?
In those cases there's a tacit understanding we have reached with the Attorney General and the Department of Justice. In cases involving provincial prosecutions in which the constitutionality of the Criminal Code is at issue, it is the lawyers from the Department of Justice who typically will defend or will intervene to defend the code.