rationale for hw.model

I'm trying to contribute to the ocsinventory-ng unix agent
whose job is to collect various informations about the machine
on which it runs ; I'm trying to make it work on *BSD.
The task is fairly easy with dmidecode, but I have to make for
its unavailability on non-i386 platforms (or on i386 where
it gives no result).
I came to use sysctl in various places and I cannot figure out
the rationale for hw.model:

hw.model = AlphaStation 255 4/232
hw.model = Intel Pentium 4 (686-class)
hw.model = SGI-IP22
hw.model = SUNW,SPARCstation-20 (TMS390Z50 v0 or TMS390Z55 @ 50 MHz, on-chip 
FPU)
hw.model = SUNW,Ultra-1 (SUNW,UltraSPARC @ 166.999 MHz, version 0 FPU)

In some cases it identifies the computer, in other cases the cpu,
and sometimes both.
What should one expect from hw.model?

Also, I'd appreciate any pointer to how I can access firmware informations
(e.g. serial number) on non-i386 platforms ... well at least on some of them.

Regards,
Thierry.