acpi0 device

Hello!

According to Absolute FreeBSD (by Michael W. Lucas, 3rd Edition, Chapter
6), acpi0 is the "first device in the system", so probably it acts as
root device of the device tree.

Sometimes acpi0 refers to the motherboard, sometimes to the case
manufacturer; it may also mention the power button.

But what does it exactly represent? Is it just a logical starting point
for the device tree, or is there some physical device behind it?

I did not find much information about this in acpi(4), which keeps being
generic about ACPI. Is there is some other reference or manpage that
deals with this?

Bye!

Rocky