code flag vs blue peter

code flag and blue peter both are nouns.

Word	Noun	Adjective	Verb	Adverb
code flag	Yes	No	No	No
blue peter	Yes	No	No	No

As nouns, blue peter is a hyponym of code flag; that is, blue peter is a word with a more specific, narrower meaning than code flag:

code flag: one of an international code of flag signals used between ships
blue peter: a blue flag with a white square in the center indicates that the vessel is ready to sail

code flag (noun)	blue peter (noun)
one of an international code of flag signals used between ships	a blue flag with a white square in the center indicates that the vessel is ready to sail