That property has effect only if Location is float.
That class is merged with CssClass and AppStyling css classes igsli_ValueLabelFloatL or igsli_ValueLabelFloatR or igsli_ValueLabelFloatT or igsli_ValueLabelFloatB depending on location of label and orientation of slider.
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