The netinstall image is a lightweight ISO (around 150MiB) that will only contain the installer, but no actual RPM packages. Sometimes, it's more convenient/faster when your ISO is on a slow connection (e.g. a virtual media using a server IPMI).
You can download it on this URL: https://mirrors.xcp-ng.org/isos/8.2/xcp-ng-8.2.1-netinstall.iso?https=1.
As with the regular installation ISO, write it on an USB media:
dd if=xcp-ng-8.2.1-netinstall.iso of=/dev/sdX bs=8M oflag=direct
Everything else is like the regular install, except that it will not offer to install from local media, only from distant ones.