Installation manual is too terse

[Richard Owlett <rowlett@cloud85.net>]

_B.5.3. Chainloading preconfiguration files_ says in part:
# More flexibly, this runs a shell command and if it outputs the 
names of
# preconfiguration files, includes those files.
#d-i preseed/include_command \
#      string if [ "`hostname`" = bob ]; then echo bob.cfg; fi

How complex can the shell command be?
Where could I find other examples?

I have a germ of an idea. I need more data before asking too 
detailed questions.