stand in vs sub

stand in is not a noun while sub is a noun.

Word	Noun	Adjective	Verb	Adverb
stand in	No	No	Yes	No
sub	Yes	No	Yes	No

As verbs, stand in and sub are synonyms defined as:

Other synonyms of stand in include fill in, substitute.

stand in (noun)	sub (noun)
	a submersible warship usually armed with torpedoes
	a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States

stand in (verb)	sub (verb)
be a substitute	be a substitute