Snap line base points
When you hold the cursor at a vertex for .5 seconds (the default setting), a small plus sign appears. This is the snap point for snap lines.
On geometry elements, you can have snap points at the start point, the end point, the midpoint (of lines), and the intersection between two elements.
There is a limit to the number of snap points that can exist at one time; when it is reached, the snap point that hasn't been visited for the longest time disappears. Change this number in the System menu: > > > tab.