What does it mean to "modify" Ubuntu?

Under the Ubuntu IPRights, it says that

You can redistribute Ubuntu, but only where there has been no modification to it.

What does a modification mean? In my case, it is just taking the server rootfs, changing a few config files, and installing firmware packages.

In my specific case, my script must extract the rootfs, chroot into it, and install packages (etc). This is cumbersome for my users, who would rather like an ISO. My project builds a custom Linux kernel and is incompatible with normal UEFI systems. So, is there a way to redistribute an image file containing Ubuntu with packages and configuration files modified so my users can just flash an ISO?