This is not an INI flag at least in RA2/YR. The game does use this string to log the sidebar location, but it is not read from the ini. It is used like this:
ra2(md).ini parsing code: GameOptions->SidebarSide = ssRight; // hardcoded Log("SideBar on %s\n", "RIGHT");
As with Tiberian Sun, there is a means within the game code of specifying the side of the screen on which the Sidebar appears (left or right) from within the RA2.INI file although this is hard coded to RIGHT and unfortunately there appears to be no way to override this. Perhaps this may be addressed by Westwood in a future patch or expansion pack as this feature would prove particularly useful for left-handed players. This keyword may work, however it is not known what its parameter may be.