In buildings that evacuate in stages, the alarm speaks twice: one signal means leave now, another means stand by and be ready. Getting them the wrong way round is dangerous, and the only authority for which is which in your building is your building's own fire procedure, not a general rule from the internet.
The usual convention
In a two stage system the continuous signal is normally the evacuation signal, meaning leave by the nearest route immediately, and the intermittent or pulsing signal is the alert signal, meaning a fire has been detected elsewhere, prepare to leave and await instruction. Single stage buildings have one signal and it always means evacuate, which is why most people have never encountered the distinction.
Why phased evacuation exists
In a tall or very large building, moving everybody at once floods the stairs and slows the people in most danger. Phased evacuation moves the floors nearest the fire first and holds the rest, then extends if needed. It only works if the people being held understand that being held is the plan, which is entirely a matter of training and signage rather than of the sounders.
Voice alarm changes the question
Buildings with voice alarm systems replace tones with spoken messages, which removes the ambiguity altogether: the system says what to do. This is why voice alarm is common in shopping centres, transport buildings and tall offices where a large proportion of occupants have never seen a fire notice. Where a voice system exists, the message is the instruction, not the tone under it.
Where to find your building's answer
The fire action notice by each call point and exit should state what each signal means and what to do. If it does not, that is a finding for the responsible person rather than a question for a search engine. A building with a two stage system and notices that describe one stage has a training problem that will show up on the day it matters.
Questions people ask about what does a continuous fire alarm mean
Does a continuous alarm always mean evacuate?
It usually does, and in a single stage system there is only one signal and it always means evacuate. In a two stage building, check the fire action notice, because the local procedure is the authority.
What should I do on an intermittent alarm?
In a two stage system it normally means alert: stop what you are doing, prepare to leave and wait for instruction or for the signal to change. Follow your building's notice rather than a general rule.
Why do some buildings use spoken messages?
Because a voice alarm removes the ambiguity between tones and can give different instructions to different areas. It is common where many occupants are visitors who have never read a fire notice there.