2) Type of MCB/Fuse/RCBO is dependent of inrush currents. Simple example would be say a welding machine, because there are high currents (inrush currents) when the unit starts you would pick a D type breaker so that it would not trip, these loads are inductive loads. A immersion heater would only require a B type breaker as you would have none of these inrush currents and is call a resistive load. So you choice breaker will be dependent on type of load inductive or resistive
3) Depending on additional protection provided by an RCd. For instance circuits in a room containing a bath and shower have to be protected by 30mA RCD, so that influences your selection of protection devices, as it's a regulation, these regulations come under the Automatic Disconnection of Supply or ADS