ah, *this* one :-)
This is a icon decorator for a 'virtual folder'. You can create this if you drag&drop a folder outside of the project into the project. Then it asks you if you want to create a linked folder (a link which links to the file folder) or if it should be a virtual folder (no physical folder).
You can get the information if you select the folder and use properties: it will show that it is of type 'virtual folder'.
In case you are not familiar with this:
- linked folder: your project contains a link to the physical folder
- virtual folder: your project contains a virtual container with links to the files in that folder.
I hope this helps?