The National Fire Protection Association (NFPA) Code Books are really legal code that can be written into governments' laws. The Code is written to show the minimum requirements to make an adequate detect-and-warn fire alarm system.
I find the NFPA Code books to be extremely difficult to apply to real life situations. Yes, the code needs to be "complied with", but it is written so that lawyers, lawmakers, and those enforcing laws can understand it.
The NFPA Code becomes easy to understand when you know what the intent of the Code is and some of the thought of the writers of the Code. That information is included in the NFPA's Handbook.
I recommend purchasing the "NFPA 72 National Fire Alarm and Signaling Code Handbook". It has the complete official NFPA Code, and included along with the Code are lots of explanations about what the Code means.
Basically, explaining what the fire alarm system should look like, the NFPA 72 Code itself is legal-talk; explaining what the NFPA means with the legal-talk, the Handbook has added comments (using terms that even I can understand) to help show what the Code means. They even use illustrations.
Yes, the Handbook is more expensive. However, to get an understanding of what is meant by the legal terms shown in the codebook, the added explanations are worth much more than the extra expense.
The Handbook is available in hardbound, in softbound, or even in electronic form for desktop or mobile devices on the official NFPA website --- https://catalog.nfpa.org.
The Handbook is where I found out what is meant in the different Classifications of the Signal Path, and how the paths are supposed to operate.