You can use it for different things. my main use for it is to project targets based on impuls legs. not always will price follow the AB=CD target. instead it often reverses before reaching point D. This tool can help you identify alternative targets. Technically speaking the shown example of usage isn't called a fib extension but a fib expansion. you can get fib extensions if you put point A' exactly back to A.
You can find the tool in your drawing toolbar by clicking the third icon and selecting "Trend-Based Fib Extension". When you click on a drawn fib extension a little options tootlbar will appear where you can customize your fib levels. Commonly used levels for expansions / extensions are: 0.382 - 0.5 - 0.618 - 0.764 - 1 - 1.272 - 1.414 - 1.5 - 1.618 - 2
Keep in mind though, it is statistically proven that Fibonacci extensions work no better at predicting where price is going to turn than any other percentage. It's more of a self-fulfilling prohecy than anything else.