Best practices
Use a panel to give people quick access to important controls or information related to the content they’re working with.
Consider using a panel to present inspector functionality.
Prefer simple adjustment controls in a panel.
Write a brief title that describes the panel’s purpose.
Show and hide panels appropriately.
Avoid including panels in the Window menu’s documents list.
In general, avoid making a panel’s minimize button available.
Refer to panels by title in your interface and in help documentation.
HUD-style panels
Prefer standard panels.
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In a media-oriented app that presents movies, photos, or slides
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When a standard panel would obscure essential content
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When you don’t need to include controls — with the exception of the disclosure triangle, most system-provided controls don’t match a HUD’s appearance.
Maintain one panel style when your app switches modes.
Use color sparingly in HUDs.
Keep HUDs small.
For the complete guidance, including worked examples and illustrations, see the canonical page: https://developer.apple.com/design/human-interface-guidelines/panels