What determines if kernel modules are auto-loaded?

[David Fleck]

I'm trying to get a better understanding of how kernel modules work in
FreeBSD - one thing at the moment I can't figure out is why some modules
(e.g., cd9660.ko, usb.ko) will be automatically loaded when needed, while
other modules (vpo.ko, snd.ko) are not.  What causes the difference in
behavior?

Is there a reference describing the use of kernel modules in detail?
Everything I've found so far is either too general (the Handbook) or way
too specific and development-oriented ("here's how to write a kernel
module that will hack someone's system!").

--
David Fleck
david.fleck at mchsi.com