public relations vs promo

public relations and promo both are nouns.

Word	Noun	Adjective	Verb	Adverb
public relations	Yes	No	No	No
promo	Yes	No	No	No

As nouns, promo is a hypernym of public relations; that is, promo is a word with a broader meaning than public relations:

public relations: a promotion intended to create goodwill for a person or institution
promo: a message issued in behalf of some product or cause or idea or person or institution

Other hypernyms of public relations include packaging, promotion, promotional material, publicity.

public relations (noun)	promo (noun)
a promotion intended to create goodwill for a person or institution	a message issued in behalf of some product or cause or idea or person or institution