Hi Filippo, it's a "filter" symbol - when that is activated by doing a ctrl+click on that selection dot area of the scene browser it will enable filtering for that item which means that any selection actions (like clicking to select, window selection, or just "select all") after that are limited to only target that category of objects.

This allows for some kinds of specific selection tricks, like for example say you have a mixture of some curves and some solids that all have that name, and you only want to select the curves only.

You can make that happen by setting the filter on that name as you show there (by ctrl+click on the selection dot area), and then go to Types > Curves and click the selection dot there - because the filter is set only the curves that have that active filter name will be targeted by that selection action.

You can set filters on multiple items, and you can clear the current filter by pushing the escape key.

Some more info here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4438.21
http://moi3d.com/forum/index.php?webtag=MOI&msg=4570.2


So for example you can set a filter on Style > Red and another filter on Types > Curves and then do a window select in one area of the screen and only the red curves that are in that window will be targeted for selection.


EDIT: ahh Pilou beat me this time! ;)

- Michael