Now that we’ve created an image with a basic installation of El Capitan and a couple of apps, it’s time to create images we can add to the NetInstall service.
As a reminder, there are three kinds of images you can create with the System Image Utility.
NetInstall Image: A network-based installer image you can use to install or update an operating system over a network connection.
NetBoot Image: A network-based operating system you start your Mac with rather than using an operating system installed on a local hard drive.
NetRestore Image: A network-based image that wipes your Mac’s hard drive and replaces it with the contents of the network image.
The NetRestore image will erase the contents of the hard drive on the Mac you install it on!!! I’ll say it again later, but DO NOT run this image on a Mac that isn’t backed up or from which you can’t afford to lose what is installed on the hard drive. It will be gone once you run the NetRestore. No fooling!
Read more here:: How to create NetBoot and NetInstall images