Use the preview window to view how a label style will appear in the drawing. When you make modifications to the label style, the preview window updates.
Previews are actually stored as DWG files in the \Data\Preview folder. If you would like to add your own preview drawings that contain specific label styles, you can place the drawings in the Preview folder.
Do not rename the Preview folder or its sub-folders. Any changes to the folder structure prevents AutoCAD Civil 3D from accessing the preview drawings.