Relations among different classes might be clear for me, however, the way using unified modeling language (UML) to represent it is something new to me. A note on this topic is worth taking to make it on my dictionary. Taken from Core Java Volume I (11th edition) - Cay S. Horstmann.
In the web environment, the user can have their browser window set to any number of sizes, and the CSS has to apply to it. Also, users can resize the page after it's opened, and the CSS needs to adjust to new constrains. This means that the browser must calculate those when the page is rendered onscreen. Relative units are one of the tools CSS provides to work for the responsive design.