substituting vs subbing

substituting and subbing both are nouns.

Word	Noun	Adjective	Verb	Adverb
substituting	Yes	No	No	No
subbing	Yes	No	No	No

As nouns, substituting and subbing are synonyms defined as:

substituting and subbing: working as a substitute for someone who is ill or on leave of absence

substituting (noun)	subbing (noun)
working as a substitute for someone who is ill or on leave of absence	working as a substitute for someone who is ill or on leave of absence