Creation of overrides (indices) files

The best place to answer these questions, cause I don't know the answer's by heart, is to check the source

so I cloned the git repo from here:
http://packages.qa.debian.org/t/tasksel.html

as in:
git clone --depth 1 git://git.debian.org/git/tasksel/tasksel.git

Now I had a look at the code, and couldn't find what I was looking for, so I checked their README

---------------------------

On startup, the tasksel program will read all *.desc files in
/usr/share/tasksel/ for information about what tasks are available. The
tasks will be presented in a simple list selection screen with their short
descriptions.

On exit, tasksel executes the appropriate command to install the selected
packages. If the -t option is given, then tasksel prints out the command
line to use to stdout instead. All other messages are printed to stderr.

To get a new task added to Debian, please file a bug report on tasksel.

Debian derived distributions can add a new .desc file to
/usr/share/tasksel/ to add additional tasks, or modify/divert
debian-tasks.desc to remove tasks.
---------------------------

Other interesting reading might be:
https://wiki.ubuntu.com/PackageDependencyManagement

I read this readme, but I still don't see how this answers my questions regarding override files.

I'm not sure what you mean by overrides. Taksel operates in a supervisory fashion to apt. The metadata is stored in the .desc files.

Eg:
$ head /usr/share/tasksel/ubuntu-tasks.desc
Task: dns-server
Section: server
Description: DNS server
 Selects the BIND DNS server and its documentation.
Key:
 bind9
Packages: task-fields

Task: edubuntu-desktop-gnome
Section: user

This is wholly outside the package dependency metadata which is stored in the control file, inside the control.tar.gz inside the .deb file...

An example override file: http://archive.ubuntu.com/ubuntu/indices/override.lucid.extra.main

Well, those are for the archive uploads I believe, and are not part of the end user system:

https://wiki.ubuntu.com/ArchiveAdministration#Component%20Mismatches%20and%20Changing%20Overrides

So to answer your question, they are maintained by hand by the FTP masters. Probably by hand. The relation to the tasksel is not an explicit one (though it is implicit), which is what confused me in your initial question.

The priority is for dependency resolution for tools such as aptitude and apt-get; if it was an "optional" package, why would it be part of the base ubuntu-desktop system? that would be nuts :). When package maintainers create new packages, normally it is created with priority: optional, unless it is critical to running a normal system
http://www.debian.org/doc/debian-policy/ch-archive.html#s-priorities

For example, you could create an override file like so (see 6.4.11):

http://qref.sourceforge.net/Debian/reference/ch-package.en.html