These are the two decisions that actually change what goes in your board, so here's the plain version.
Type AC only sees AC faults. That's fine on a lighting or heater circuit, but it won't reliably pick up the DC component that comes off modern gear.
Type A sees both. You need it on EV charger circuits, inverter washing machines and dishwashers, variable speed split systems and ducted units, heat pumps, and solar or battery systems. Type A devices cost more than Type AC, so Glenn picks per circuit based on what's actually plugged into it.
An RCD covers several circuits at once. Cheaper up front, but when it trips it takes the lot with it and you're guessing which circuit caused it.
An RCBO is a breaker and an RCD in one device, on one circuit. Only the faulty circuit drops, so you know straight away where the problem is. Most boards Glenn upgrades these days go to RCBOs for that reason.
If the board is full, has ceramic fuses, or the main switch and wiring are past it, adding devices isn't the answer and a switchboard upgrade is. Glenn will tell you which one you're looking at once he's had the cover off.
