I've been playing around with the autocomplete system for a while now, digging up what I can, which hasn't been much.

In particular I want to be able to better control the autocompletion menu and use it for my own purposes.

I found the "FE`" level symbols and their corresponding front-end packets, like FE`AC, FE`OC, FE`FC, etc., but can't figure out what they do.

When I use them (e.g. by FE`FC["Print"]) all that happens is I get an "INTERNAL SELF-TEST ERROR", which I naïvely suspect is because the AttachedCell popup window that houses the completions isn't available.

I understand this is undocumented functionality and using it is morally equivalent to taking candy from little kids, but I'd still like to know how it works and don't think anyone outside of the WRI can steer me in a useful direction.

So can anyone help me out here?