This tool is similar to a balloon. The difference is that callout allows user to extend its point from a specific location to a more customized placement.
All options available for a text inside a callout, may be set in Style properties dialog:
Allows you to change the text color, font size, set bold or italic font.
Where the text is typed.
Toggles the visibility, color and opacity of a background for the text box.
Toggles the visibility of a border for the text box. Can also change the border color.
Enables/Disables text wrap within the text box.
In Coordinates properties dialog you can set precisely the position of the point and container of a callout vertically (by setting the price) and horizontally (by setting the bar number):
Allows for the precise placement of the callout's point (Price 1) using a bar number and price.
Allows for the precise placement of the callout's container (Price 2) using a bar number and price.
In Visibility properties dialog you can switch the Callout tool displaying on charts of different timeframes: