Add a decoration to the selected item in the artboard.
Zaxxon is writing a data sheet plugin. He wants to show certain information to the user about the selected scene node(s) depending on certain data the user added. The user selected the layer and set properties in the panel. When the user selects the item again the icon(s) appear in the corner of the item.
It may make more sense that these decorations would be shown whether the plugin panel is open or not. If plugin panel is open the panel can possibly show that information in some way but if the plugin panel is not shown then it is a way to indicate plenty of information to the user without the panel being open.
For example, another use case. Your plugin panel is closed and you are selecting elements. You select a repeat grid. Let’s say the user uses a plugin that updates that repeat grid with actual data. When the user selects that element, a request is made behind the scenes and it checks if there is new data available. The plugin shows that the data is out of date and so it shows an icon decoration on that element to show that the data needs to be refreshed. In some circles I think they call it badges. This is just an example.