An Ellipse is typically used to highlight or circle areas on a chart. For example, price gaps, peaks and areas or overbought and oversold conditions.
In Style property dialog it is possible to change the appearance of an ellipse:
Sets the color and thickness of an ellipse border.
Toggles the visibility, color and opacity level of an ellipse background.
In visibility properties dialog you can switch the Ellipse tool displaying on charts of different timeframes:
If you wish to draw a circle, hold the Shift key pressed while drawing: