sustenance vs bread and butter

sustenance and bread and butter both are nouns.

Word	Noun	Adjective	Verb	Adverb
sustenance	Yes	No	No	No
bread and butter	Yes	No	No	No

As nouns, sustenance and bread and butter are synonyms defined as:

Other synonyms of sustenance include keep, livelihood, living, support.

sustenance (noun)	bread and butter (noun)
the act of sustaining life by food or providing a means of subsistence	the financial means whereby one lives
a source of materials to nourish the body
the financial means whereby one lives