What is the relationship between COREBOOT and SEABIOS

I don't know what kind of complicated answer you are looking for, here is my simple, but lengthy and anecdotal, attempt:

Consumers have been using IBM PC clones for a long time. These had a firmware for platform initialization called BIOS, which at some point was reverse engineered so that other manufacturers than IBM could start to produce compatible machines. Other computer systems use different firmware, like Itanium, where EFI was developed which later became the new standard for Macs and PCs with Intel processors (TianoCore). Coreboot, initially named LinuxBIOS is an attempt to remove most, or all if possible, proprietary code to boot your Linux kernel, which was the initial goal. At some point members of the project at the time thought that it would be more flexible to support mechanisms other than just loading a Linux kernel. Replacing the original firmware of your computer with Coreboot seemed difficult, even among Linux users, and if you are successful you would be stuck with only booting Linux. Running Windows on Coreboot may not be what most have in mind, but it has its niche. A better example would be users who are more familiar with BSD. And of course there are many more: Minix, Haiku and so forth. If you want to boot any of these you must choose a "payload" that provides an interface that these operating systems expect.

Further reading and materials:

• Matthew Garrett: ACPI, kernels and contracts with firmware
• Depthcharge: The Chrome OS Bootloader
  • There was a nice and simple diagram on the Chromium project website comparing Chome OS devices to PCs and explaining what the engineers removed from the boot process to achieve short boot times. Unfortunately I can't find that right now.

I'm not a firmware engineer and I don't work or contribute to any of these projects. I'm just someone else with over a decade of Linux experience.

If you want to ask an engineer you could ask MrChromebook on Reddit or ask at the respective mailing lists.