The SteelSeries WoW mouse sends non-standard key codes to the operating system. If you want to use these peculiar key codes in a script, you have to assign a name to them. That's what this keyword is for.
For example, suppose you want to program the SteelSeries mouse to generate "Y" when you press a button, and you want to use that button to trigger a hotkey. Here's what you do.
1. Program the mouse button to generate "Y".
2. Press the mouse button while looking at "Last key press" on HotkeyNet's screen. Note the virtual key code and whether the key is extended.
3. In this example (using "Y") the SteelSeries mouse generates virtual key code 89 extended.
4. In your script, define the key as follows. You need to invent a name for this non-standard key such as "SteelSeriesY."