The first, left-most column in the structure tree is an indicator, or traceability column. This column can be used by views loaded to the right to indicate specific items in the structure:

In this example, the red slice of the pie is selected and the items counted in that slice are highlighted in red in the indicator column to the left.

If there are indications under a collapsed structure, the indicator column will include a number of the total indications in the collapsed structure.

When creating an own extension, you can utilize this indicator column using the following methods.

The IswItemHost has a method named IndicateItems(IswItems items) that indicates the provided items in the structure tree. 

The host is provided in the method SetHost that is in the IswItemViewContent interface which is called from the swExplorer and can be implemented in the code-behind of the content xaml file.