system vs plan of action

system and plan of action both are nouns.

Word	Noun	Adjective	Verb	Adverb
system	Yes	No	No	No
plan of action	Yes	No	No	No

As nouns, plan of action is a hypernym of system; that is, plan of action is a word with a broader meaning than system:

system (noun)	plan of action (noun)
instrumentality that combines interrelated interacting artifacts designed to work as a coherent entity	a plan for actively doing something
an ordered manner; orderliness by virtue of being methodical and well organized
a group of physiologically or anatomically related organs or parts
the living body considered as made up of interdependent components forming a unified whole
a complex of methods or rules governing behavior
a procedure or process for obtaining an objective
an organized structure for arranging or classifying
a group of independent but interrelated elements comprising a unified whole
(physical chemistry) a sample of matter in which substances in different phases are in equilibrium