AOSX-13-000005 - The macOS system must conceal, via the session lock, information previously visible on the display with a publicly viewable image.
A default screen saver must be configured for all users, as the screen saver will act as a session time-out lock for the system and must conceal the contents of the screen from unauthorized users. The screen saver must not display any sensitive information or reveal the contents of the locked session screen. Publicly viewable images can include static or dynamic images such as patterns used with screen savers, photographic images, solid colors, a clock, a battery life indicator, or a blank screen.
This setting is enforced using the 'Login Window Policy' configuration profile.