The Sub tag is useful in situations when you need part of your text to be a little lower than the rest of the text.
For example-

H<SUB>2</SUB>0

Will look like this-

H₂0

The Sup tag is useful in situations when you need part of your text to be a little higher than the rest of the text.
For example-

E=mc<SUP>2>/SUP>

Will look like this- E=mc²

You could try mixing the two tags for a crazy auction title!