My interface (RME UCX) has 2 1/4 plugins that have a hi-z setting. I'm still trying to work out when to set this setting in the software for the interface.
 
So, I know to use the hi-z input when I'm playing my electric guitar and set the input to hi-z in total mix.
Do I do the same when plugging in an acoustic-electric guitar with a battery powered microphone inside the guitar?
What do I set it to for an input from a direct box?
When do I not set it to Hi-Z?

I notice that the Hi-z spec for the RME UCX is 470 K ohms.
Some direct boxes have higher values than this. Should I introduce a direct box in the middle in order to take advantage of a higher value like 800ish K ohms?

Sorry for the beginner's questions.

I found two answers...
 
one here...
 
and one is this...

• Basically, you want your Z (impedance) either equal or much      greater than the output Z of the box sending the signal. Usually      for high-bandwidth signals (not analog audio, but definitely digital audio) you want the Z's equal, and they will usually be 50 or 75 ohms.
•       For typical audio, you probably want to send into the highest Z you can. Higher Z on the input device will make you more noise      sensitive, so if you have noise pickup issues try the low-Z setting. Low-Z will eat more power from your output device, so may reduce battery life, though I would think not by too much. Using low Z may reduce the volume you hear, so if it seems like the volume is too low, go back to the high Z setting. If unsure, use high-Z input since the output device may not be designed to work well into low-Z and may distort.
•       If you have an electric guitar w/piezo pickups without an internal amp, use high Z for sure. If there's an internal amp of some kind, either will probably work, may well be designed for low-Z. would      need to see the manuals to be sure.
•       470k is plenty of input impedance - no need to jack that up with      anything else.
•       Hope that helps-

The other is this...
http://www.gollihurmusic.com/faq/30-OHMS_AND_IMPEDANCE_FROM_THE_INPUT_PERSPECTIVE.html