red herring vs digression

red herring and digression both are nouns.

Word	Noun	Adjective	Verb	Adverb
red herring	Yes	No	No	No
digression	Yes	No	No	No

As nouns, digression is a hypernym of red herring; that is, digression is a word with a broader meaning than red herring:

red herring: any diversion intended to distract attention from the main issue
digression: a turning aside (of your course or attention or concern)

Other hypernyms of red herring include deflection, deflexion, deviation, divagation, diversion.

red herring (noun)	digression (noun)
any diversion intended to distract attention from the main issue	a turning aside (of your course or attention or concern)
a first draft of a prospectus; must be clearly marked to indicate that parts may be changed in the final prospectus	wandering from the main path of a journey
a dried and smoked herring having a reddish color	a message that departs from the main subject