That is applied to SPAN element and it is merged with igtab_XXMirrorCenter. That is used only for combination of vertical text with top/bottom locations of tabs.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2