This opens the Class Name(s) dialog. Use the Class property to specify a class name for the field label.

Many different types of controls have field labels. The CSS styling varies for these labels depending on the control type and the style name chosen for the component. You can set the component styling under the Style Name property on the UX Properties page under the UX properties section. The Class property allows you to apply the CSS styling used on other types of controls to the Label control. Below are a few examples of different CSS class names using the iOS7 Style name and applied to a Label control.

The styling, size, and font used in this case are set in the Field Label > Style property.