Evacuation of a high rise building - means of escape:

Evacuation of a high rise building is likely to be split into two or more phases.  Initially, when the alarm activates (from a single device) there will be an investigation period when the building staff check the cause of the signal and when no evacuation signal is given.  If there is no fire then the system is reset and no one is disturbed.  If a fire is confirmed, then evacuation of the building starts.  Sometimes it is just the fire floor that evacuates initially, sometimes it is the fire floor and adjacent floors.   After the initial evacuation other floors in the building are evacuated, sometimes in pairs moving up the building or sometimes all together. This is referred to as phased evacuation.