national flag vs emblem

national flag and emblem both are nouns.

Word	Noun	Adjective	Verb	Adverb
national flag	Yes	No	No	No
emblem	Yes	No	No	No

As nouns, emblem is a hypernym of national flag; that is, emblem is a word with a broader meaning than national flag:

Other hypernyms of national flag include flag, allegory.

national flag (noun)	emblem (noun)
an emblem flown as a symbol of nationality	special design or visual object representing a quality, type, group, etc.
	a visible symbol representing an abstract idea