This mode of xterm and mintty supports enhanced application control over modified keys (not function keys).
Control sequences to terminal:
ESC[>4;2m enable "modifyOtherKeys" mode
ESC[>4m reset "modifyOtherKeys" mode
(optional: ESC[>4;0m disable "modifyOtherKeys" mode)
Control sequences sent from terminal if a "normal" (non-function) key is combined with modifiers:
<unicode> is the decimal Unicode value (considering Shift state)
<mod> is the usual modifier code (5=ctrl, 6=ctrl-shift, 7=ctrl-alt, 8=ctrl-alt-shift)
Encoding shift is optional (xterm does it, mintty does not).
Control-a ESC[97;5u (optional, ^A would be fine here)
A use case is input support for accented characters as provided by the Unicode editor mined:
Control-, c will enter ç etc.
This would be good to support especially in a dedicated multi-lingual terminal.