What is bps/cpg?

[Kevin A. Pieckiel]

I was reading the man page for bsdlabel, specifically looking for what the
bps/cpg value for a new filesystem should be.  The man page states:

bps/cpg
             For 4.2BSD file systems, the number of cylinders in a cylinder
             group.  For LFS file systems, the segment shift value.  Defaults
             to 16 for partitions smaller than 1GB, 64 for partitions 1GB or
             larger.

Okay.  Sure.  That leaves a few questions:

1. What is the purpose of cylinder groups?
2. What are "reasonable" values for this parameter?
3. Why, if the default is 16 or 64, did sysinstall create my various
   filesystems (on various machines) with values of 8, 28552, or 32776?

I'm just trying to understand why this value is set to these numbers by
sysinstall, and to what I should set this value when I add a HDD to a
computer.

Kevin