[yocto] variable override syntax, where is it documented?

[Bernd]

I just found out that I can ovverride the line

hostname = "${MACHINE}"

in base-files simply by assigning a value to

hostname_pn-base-files

in my configuration file and it will re-assign it with a higher
priority than "=".

Now I wonder where this kind of black magic is documented, I don't
seem to be able to locate any document that explains the "_pn-" syntax
and I wonder if there are even more such tricks that could be useful
and am looking for a document that explains them all. Is there such a
thing?

- Bernd