The Underline elements state that the enclosed text should be rendered, if practical, underlined.

The <U>main point</U> of the exercise...

would be rendered as :

The main point of the exercise...

<U> can also take the CLASS, ID and STYLE attributes to allow style sheet definitions to be applied to it. For more details of these attributes, see the Style Sheets topic.

<TT> Information and Character Formatting Elements Overview