This blending mode allows the content behind a window to show through. Some interface elements, such as menus, sheets, and sidebar tables, adopt this mode automatically, although you can disable it if necessary.

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The system offers two ways to blend background content:
- Behind-window blending.
- In-window blending.

macOS: Background Blending Modes

Apple Human Interface Guidelines

macOS: Behind-window blending.

 This blending mode is adopted by elements that reside on windows and allows window content residing behind the element to show through. Toolbars often implement this mode, and hint at window content that scrolls beneath the toolbar.

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