flanker vs flanker back

flanker and flanker back both are nouns.

Word	Noun	Adjective	Verb	Adverb
flanker	Yes	No	No	No
flanker back	Yes	No	No	No

As nouns, flanker and flanker back are synonyms defined as:

flanker and flanker back: a back stationed wide of the scrimmage line; used as a pass receiver

flanker (noun)	flanker back (noun)
a soldier who is a member of a detachment assigned to guard the flanks of a military formation	a back stationed wide of the scrimmage line; used as a pass receiver
a back stationed wide of the scrimmage line; used as a pass receiver