P.A. vs communication system

P.A. and communication system both are nouns.

Word	Noun	Adjective	Verb	Adverb
P.A.	Yes	No	No	No
communication system	Yes	No	No	No

As nouns, communication system is a hypernym of P.A.; that is, communication system is a word with a broader meaning than P.A.:

P.A.: an electronic amplification system used as a communication system in public areas
communication system: a system for communicating

P.A. (noun)	communication system (noun)
an electronic amplification system used as a communication system in public areas	facility consisting of the physical plants and equipment for disseminating information
	a system for communicating