I don't know how correct this is, but my instructor was emphatic about us using the term "torting" when talking about the fruit/custard fillings placed in between cut layers and that "filling" the cake is the icing between the actual layers.
Hmm. I think torting just refers to cutting a layer into sections, or into separate cakes. The word torte means a rich cake and doesn't have a definition as a verb,technically, that I could find. I think we've created that term as cake decorators. I don't think it means the filling you place between the cut layers, but I could be wrong, too. I think the filling is anything you place between layers, including between cut layers and that it can be icing or whipped ganache or fruit or custard fillings or whatever, but it does seem that most people call the icing or frosting that is used between layers "icing"and other non-icing fillings "filling".
I'll shut up now.