ICO is a file format specifically designed for storing computer icons on Microsoft Windows. An ICO file can contain multiple images at different sizes and color depths within a single file, allowing the operating system to display the appropriate icon resolution depending on the context (e.g., desktop, taskbar, or file explorer). It is also commonly used for website favicons. Sample ICO files are used to test icon rendering on different operating systems and browsers. Developers use them to verify that the correct size is selected for display and that transparency masks are properly applied. They are also used to test tools that generate multi-resolution icon sets from a single source image.