The option "Test this media" actually sets the
rd.live.check dracut command line option, which in turn triggers Anaconda, the installer, to test the installation media, by running the systemd
checkisomd5 one-shot service. The definition for this service is:
Description=Media check on %f
ExecStart=/bin/checkisomd5 --verbose %f
checkisomd5 command (see here) is provided by the
isomd5sum package in Fedora:
[root@fedora30 /]# dnf provides checkisomd5
Last metadata expiration check: 0:02:27 ago on Tue 03 Sep 2019 09:31:46 AM CEST.
isomd5sum-1:1.2.3-5.fc30.x86_64 : Utilities for working with md5sum implanted in ISO images
Repo : fedora
Filename : /usr/bin/checkisomd5
So let's check the source:
rpm -ivh https://download.fedoraproject.org/pub/fedora/linux/releases/30/Everything/source/tree/Packages/i/isomd5sum-1.2.3-5.fc30.src.rpm
tar -xvf 1.2.3.tar.gz
Going through the source, you'll find a few references to the
supported flag, which apparently means:
Indicate that the image will be written to a "supported" media, such as pressed CD. On Red Hat-based Anaconda installers, this bypasses the prompt to check the CD.
which agrees with the man page at https://linux.die.net/man/1/implantisomd5.
So, "Supported ISO: no" means that this image wasn't flagged with the "supported" flag at creation time. This will make Anaconda show the prompt to check the CD.