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.

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.

macOS: In-window blending

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

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macOS apps use translucency and blurring to evoke a sense of depth, by enabling views and controls to hint at content residing in the background. They also use vibrancy, a subtle effect that dynamically blends the foreground and background colors using a careful balance of lightening and darkening techniques. This combination helps make foreground content stand out against any background.

macOS: Translucency

Apple Human Interface Guidelines

- Be prepared for vibrancy. 
- Enable and disable vibrancy in your app when appropriate.
- Prefer system-provided elements and colors.
- Use template images.

macOS: Translucency and Vibrancy

macOS: Background Blending Modes

The system-provided materials define the amounts of translucency, blurring, and vibrancy applied to your interface. The system provides many standard materials for you to use, each one with a designated purpose.
- Choose materials based on their semantic meaning.
- Use system colors with system materials.

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