Can I just ask a really basic, possibly irrelevant and pedantic question, please?

Why is the horizontal line in this sketch fully defined? The glossary defines fully defined as "cannot be moved. Fully defined sketch entities are shown in black."

But grab any of the 3 vertices and drag them and the 'fully defined' line will move. Ok the end point of the line is blue, so you can easily figure out that only the length of the line can change. (You can make it flip direction.) But at the end of the day that line has 1DoF just like the dimensioned line.

Is anything in the underlying SW behaviour here useful to understand?