Image for Mac devices can either be created using Apple System Image Utility or any third-party image creation utility such as AutoDmg.
For more information, see https://support.apple.com/en-in/guide/system-image-utility/welcome/mac
The image created using any of the methods will have an NBI extension. The NBI extension folder includes many files as shown in the image:
Following are some of the important files:
NetBoot.dmg: This file contains the disk image file.
NBImageInfo.plist: This file contains configuration
booter: Bootstrap file that is downloaded via TFTP image configuration and in-turn chain loads the other required files.