Is there a guide as to what does what?
Nope. This is super-secret-proprietary-information... that's also pretty useless in most cases these days. It's basically just a matter of setting what specific model of
console you have (24/48, 48/96, 72/144, 125, or 250). If you figure out the super secret magic, then the only case where you actually benefit is if you happen to have a 125 and can figure out how to make it a 250 without breaking anything. That would give you a still very limited (by modern standards), but slightly higher
channel count. If you have any other version, then you'd either be adding faders you don't physically have or disabling ones you do have.
There's also really nothing
practical you can do in terms of software unless you have the extremely specialized skillset required to read the completely unique
operating system from the internal EEPROM and then reverse-engineer it. Even ignoring that massive hurdle, my understanding is that
ETC stopped developing the
Express software when they did because they had already squeezed every ounce of performance possible out of that hardware. There's not much more that could have been done without a big hardware redesign...
..which leads me to my actual suggestion: if you're looking for a project, think about starting with a hardware redesign. I've fixed
Express consoles before, and the facepanel electronics are pretty simple. It's a bunch of analog multiplexing for the faders, and a pretty standard keyboard matrix (switches and diodes arranged in row/column format). It would be relatively easy to remove the
processor assembly "brick" and replace it with something more modern... maybe based on
Arduino or Raspberry Pi. You could theoretically put together something that would allow you to use it as a
wing via
OSC to control modern
console software such as Nomad. It would still be a whole lot of work, but it would be a much better learning experience and way more
practical than trying to reverse-engineer something that's been obsolete for longer than some (many?) of our younger CB members have been alive. You may even be able to sweet talk someone in phone support into sharing the schematics for the facepanel boards and save the work of following traces and figuring out which pin does what on the connection between the facepanel and the CPU "brick".
Oh, and on the off chance you haven't figured this out already: if you happen to have a 24/48, then the bottom row of faders can act as submasters when you
switch to single-scene mode.