We all like Fully-Defined sketches. There’s no room for ambiguity. The sketch and resulting features are more stable. Rebuild times are improved. SolidWorks likes fully-defined sketches as well. And, when you’re editing a sketch, SolidWorks is kind enough to tell you when your sketch is fully defined. However, sometimes it will tell you the sketch is fully defined when it’s “fully defined enough,” meaning there may be under defined bits in it. I am, of course, referring to the endpoints of construction lines. However, I’ve recently learned that there are other instances when a “fully defined” sketch is not so.
The lines I created using the On Edge sketch relation were considered fully defined until I dragged the endpoints of them. Once I started dragging them, SolidWorks recognized that I did not want the endpoints tied down so it became under defined. These are not the only examples of this kind of behaviour, either.
This is very similar to what you would see in an assembly when you use a Limit Angle or Limit Distance mate. The parts can move by dragging them, but they remain fully defined. Interestingly enough, with the Free Drag option of the new Slot mate in SolidWorks 2014, it does not consider the part’s position to be fully defined.
So much for the simple rule “if it’s blue it moves for you.”