Good question, Silvio. That's almost unknown feature, and it's an extra help. If you have several objects (all must be curves, not rectangles, ellipses nor text), all have "red nodes" except one, which have blue nodes. This shows the order of the objects. The upper object is shown as blue. Why? remember that on several shape effects (such as Trim, Combine, etc) the order of the objects or the order of the selection determines what object trim other, or what color and other atributes will have the final object. The Blue Shape is the Reference Object (the last selected object or the object in front of the others). If you change the order of the objects, the Blue object will change. Also, remember that you can select objects individually, if you hold Shift, and that determine the order. The last selected object will be the reference object.