"mode" option in live config

[Michael AU]

In live build and associated packages there is an option in the config
file for "mode". It used to have Debian. Progress, and I think Ubuntu.
I am trying to find the files that are associated with this option so
I can look at how each option differs from the others. Does anyone
know which particular files live build "reads" when it comes across
the "mode" option?
Cheers.
Michael.