Need detialed info on preseeding

[Richard Owlett <rowlett@cloud85.net>]

I'm following the example preseed.cfg given at 
http://www.debian.org/releases/squeeze/example-preseed.txt .

It gives no detailed information on correct usage of
d-i pkgsel/include string ...
AND
d-i preseed/late_command string ...

I unsuccessfully attempted to use the first. I suspect the 
second may be more appropriate.

Where are these documented?