What does rd.break boot option do?

What does rd.break boot option do? This seems alternative for init=/bin/bash, but how does this actually works? Links to the documentation and source code?

And what the hell this means? AFAIK after GRUB kernel always has control of the system.

The rd.break parameter interrupts the boot process before the control is passed over to the kernel. At this point, when you run the passwd command to do the password reset, the associated shadow file (/etc/shadow) is modified with an incorrect SELinux context. The touch /.autorelabel command creates a hidden file named .autorelabel under the root directory. On the next boot, the SELinux subsystem will detect this file, and then relabel all of the files on that system with the correct SELinux contexts. On large disks, this process can take a good amount of time.

Source: rhel - What does "touch /.autorelabel" do when we reset the root password in Red Hat Enterprise Linux? - Unix & Linux Stack Exchange

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