NEC = National Electrical Code

The National Electrical Code (NEC), or NFPA 70, is a United States standard for the safe installation of electrical wiring and equipment. It is part of the National Fire Codes series published by the National Fire Protection Association (NFPA). "National Electrical Code" and "NEC" are registered trademarks of the NFPA. While the NEC is not itself a U.S. law, NEC use is commonly mandated by state or local law, as well as in many jurisdictions outside of the United States. [1] The NEC codifies the requirements for safe electrical installations into a single, standardized source.
http://en.wikipedia.org/wiki/National_Electrical_Code

What's the difference between power limited cables and non-power limited cables?
The difference between power limited cables and non-power limited cables comes down to which sections of NEC they comply with. Non-power limited cables is a fire alarm circuit powered by a source that complies with NEC sections 760-21 and 760-23. Power limited cables is a fire alarm circuit powered by a source that complies with section 760-41.
http://www.awcwire.com/FAQ-Fire-Alarm.aspx