Hello Hightower
What page 55 is referring to is the issue of converted buildings used for sleeping accomodation purposes.
A high rise block of flats is a purpose built construction - that is to say it was designed & built as a block of flats.
A large victorian house converted into student lets is not purpose built (i.e it was originally a house for a single family house hold)
Where conversions have taken place and havent gone through building regulation approval it can be difficult (and in some cases impossible) to guarantee the level of fire seperation in the building. Therefore we require AFD in the common areas and within the flats themselves. The reason being that if a fire does break through into the common areas from one of the flats, for whatever reason, it will be picked up by the detection in the common areas.
In purpose built accomodation such as a high rise block which has gone through BR approval we can guarantee the fire seperation and levels of fire resistance within the building and thus do not require AFD in the common areas /escape routes . The theory being that a fire should be contained in each individual flat because they have been constucted to a certain fire resisting standard (60 minutes for flats in high rise blocks)