It’s not a separate file for the undo/redo keys if that’s what you’re thinking. You add that code to the bottom of “StartupHotkeys.TXT” file in the directory that I mentioned (Zbrush 4R8 > ZStartup > HotKeys), then save the file so it looks like the following…
//ZBrush Hotkeys
[BRUSH:MORE ITEMS,98] // B
[BRUSH:STANDARD,5447796] // Popup+T
[BRUSH:SMOOTH STRONGER,5447795] // Popup+S
[BRUSH:MOVE ELASTIC,5054565] // Popup+E
[BRUSH:MPOLISH,7151728] // Popup+P
[BRUSH:CLIPCURVE,4399203] // Popup+C
[BRUSH:CLAYLINE,4399212] // Popup+L
[ZPLUGIN:MISC UTILITIES :<<BRUSH,91] // [
[ZPLUGIN:MISC UTILITIES :BRUSH>>,93] // ]
[EDIT:UNDO,634] // Ctrl+Z
[EDIT:TOOL:UNDO,634] // Ctrl+Z
[EDIT:REDO,890] // Ctrl+Shift+Z
[EDIT:TOOL:REDO,890] // Ctrl+Shift+Z
Hope that helps.