and so on vs and so forth

and so on and and so forth both are adverbs.

Word	Noun	Adjective	Verb	Adverb
and so on	No	No	No	Yes
and so forth	No	No	No	Yes

As adverbs, and so on and and so forth are synonyms defined as:

Other synonyms of and so on include etc., etcetera.

and so on (adverb)	and so forth (adverb)
continuing in the same way	continuing in the same way