How can I force boot from alternate drive with boot.config?

[Peter Steele]

>>> So, more precisely, if I wanted to boot from drive 1, I'd use this?
>>> 
>>> 1:ad(1p3)/boot/loader
>>
>>Yes, unless there are more bugs hiding. :-) I fixed a few in August last year.
>
>Well, I'll give it a try and let you know if I find new bugs... :-)

I just tried this and it works as advertised--thanks. One question though: Why does this string list the device number twice? The man page describes it as

bios_drive:interface(unit,[slice,]part)filename

where bios_drive is the "drive number as recognized by the BIOS.  0 for the  first drive, 1 for the second drive, etc.", and unit is the "unit number of the drive on the interface being used. 0 for the first drive, 1 for the second drive, etc."

This sounds like it's describing the same thing, but not exactly, but I've always used the same value in both fields and it's always worked. Is there a case where these values might be different? In the test I just did I booted from the fourth drive of a four drive system using

3:ad(3p4)/boot/loader

I know my hardware and knew ad10 mapped to the fourth drive and would be referenced as drive 3 in this context. But how would I determine this generically? For example, given something like /dev/adN, how do I know what number I'd use for this drive in boot.config?