Hmm. CH+/- are not used now. I'll think about this. Remember you can also use A (letter) key to switch tracks.
To play all files in a directory: long click on a file, you will see a menu. Also, you can select the first file in a directory and press Play |> key. This action creates a playlist, so you can use >>| and |<< keys to navigate.
Ok may be not CH +/- , they may be need in future use. However letter A is not exist in Harmony app.

Here is a list if keys I can program and they least likely use in other apps :
-.0123456789
Keys with FN button marked yellow: on top of 5 6 7 8 - A B C D, on top of ★ - REC button
Thanks for information about play list of files. I enjoy your player even more now
Is it possible to memorise chosen audio track when play bunch of similar files?
For example I have all seasons of Futurama with RU and ENG tracks. RU starts as default but I want to watch all of them in ENG now. I need to switch track every time new one starts which is quiet uncomfortable especially when RU track much louder...
Thanks