Not really ambiguous David when it comes to flats.
Its simply a judgement as to whether, in the event of a fire in any one flat, the building is safe such that apart from the flat involved in the fire, the occupants of other flats may stay in their flats instead of evacuating the building.
This judgement comes from comparing the design, nature and layout of the building with the standard design benchmarks for purpose built flats, which are designed with sufficient compartmentation and fire protection measures to make this "stay Put" policy a safe option for most forseeable fires that may occur. There have been design documents of this type since the second world war right up to current building design documents such as the Building Regulations Approved Document B.
However buildings that were designed to be used on this basis can suffer over the years as workmen and contractors put in new wiring, pipework, heating etc and knock holes through the solid compartment walls and floors thus creating passages for fire and smoke to spread.
Generally purpose built blocks of flats should not need any form of fire detection and alarm system in the common areas, each flat should have its own stand alone system of mains powered domestic smoke alarms. Depending on the layout it might even be a single alarm in the entrance lobby to a flat where the bedrooms are closer to the flat entrance door than the lounge or kitchen.
If because of design weaknesses it considered necessary and appropriate to provide a fire detection and alarm system in the common areas to support a full evacuation strategy, then it should not be a system based on domestic smoke alarms, but a full system designed, installed and maintained to B5839-1. If people need to be evacuated from the building then this alarm needs to be sufficiently audible to wake them up at night, so a sound pressure level of 75dB at the bedhead would be required. In such circumstances often each flat still has its own self contained domestic smoke alarm but this is supplemented by a full detection nd alarm system in the coomon areas, with smoke detectors in staircase and lobbies, and a heat detector and sounder in each flat.
