I had an interim defended hearing too, and I also struggled to understand exactly what the difference is. My understanding though was that more time is allocated than a regular mention and that there is more scope for decisions to be made, including cross examination of a witness (single expert, or otherwise) if necessary. Whether that's applicable to all interim defended hearings, I can't say for sure. I suspect it may just be a general 'category' of hearing and that the judge/registrar will have made prior orders or at least made it clear what it is that will be determined at the hearing..?