Look at BS 5839 Part 6 - it describes in great detail the difference between a Grade A and Grade D system, and why those different grades are required in HMOs, dependent on the size and occupancy of the property.
Whilst all HMOs must be protected by automatic fire detection, human beings often detect fire before automatic detection has been activated. So we need to consider how we manually raise the alarm prior to detector activation in HMOs.
So a small two storey HMO may only need a Grade D system, because the shout of fire could be enough as a means of manually raising the alarm prior to automatic detection being activated. In a three storey property the shout of fire as manual means of raising the alarm is unlikely to be heard - hence the need for a Grade A system incorporating manual call points
But again it comes down to risk assesment, because I could have a very large two storey property where the shout of fire might not be heard in all habitable areas, and therefore I may need to consider the installation of a Grade A system.
The control and indicating equipment aspect of a Grade A system is there for several reasons. The bigger or more complex the premises gets it follows that there is a greater need for system reliability and the ability to monitor faults. Also the CIE provides power to components such as MCPs and Fire Detectors. It also allows the fire and rescue service to locate the point of activation (whether that be a zone, or an actual detector head)