substituting (Noun) — Working as a substitute for someone who is ill or on leave of absence.
substituting (Verb) — Put in the place of another; switch seemingly equivalent items. ex. "substitute regular milk with fat-free milk"
substituting (Verb) — Be a substitute. ex. "The skim milk substitutes for cream--we are on a strict diet" ex. "The young teacher had to substitute for the sick colleague"
substituting (Verb) — Act or speak as a replacement (for someone or something).