Read the dictionary definition of sub. All definitions for this word.
1. a submersible warship usually armed with torpedoes
2. 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
3. be a substitute
1. The young teacher had to substitute for the sick colleague
2. The skim milk substitutes for cream--we are on a strict diet