To make sure they cover all possibilities, the NFPA (National Fire Protection Association) uses legal type language to say anything. This is because the NFPA publishes what will become legal documents. Sometimes they use catch-all words that have many meanings.
The word "Signal" is a catch-all word that means a signal is the act of sending a message, or possibly the word signal means the contents of the message is a signal. For non-fire alarm uses of the word signal, a signal might mean "The person on standing on one hill signals another man on another hill using flags"; a message (signal) is sent using flags, or the coded message (signal) sent using the flags said "the mobile phone isn't working".
For fire alarm uses, signaling the occupants of a building could mean sounding the evacuation. The signal could be the sound itself, or it could be the contents of the evacuation message, like "Fire - Third Floor".
Then again, on the SLC (Signaling Line Circuit), the signal could be the act of sending a message along the wires, or the signal could be Alarm, or Supervisory, or Trouble.
In a fire alarm system, to understand the exact meaning of the word signal, the context of the word (where it is used) has to be taken into account.