What do icons mean on master slide layers?

The little boxes to the left of the layer name are thumbnails of the layer.

Hi Walt. I am actually referring to the layers above the base layer, in the master slide view. Only the base layer has a thumbnail.  There must be a meaning for the difference (seen in my attached image), or it is a bug. 

It's hard to illustrate, because when a layer is selected, it's hard to see its thumbnail. These pictures are taken from the slide master, but it works the same in the slide view.

The attached images may show more clearly.

Thanks Walt. I do see the thumbnails. I've determined that if there is no content on the slide, there is a horizontal line in place of a thumbnail. The slides in my master have triggers but no content. My confusion came when I saw a mix of square thumbnail boxes and horizontal lines (seen in my attached images). If I had no content on any of the layers, why did most layers have a thumbnail box like they did? The only difference between the layers with thumbnail box and layers with horizontal line is the transparency of the layer's background color.  Makes no sense but now I know these thumb/line icons do not matter for what I am doing.  
Thanks for your help! 

You're welcome. PIAWYC (Pass it along when you can).