Snap tolerance
The snap tolerance is a distance which determines whether the beginning or the end of a new segment can be snapped:
Snap/split will only be allowed if the distance between the beginning or end coordinate of a new segment and the coordinate of an existing node or any segment to which you want to connect is smaller than the specified snap tolerance.
When working with a digitizer, a snap tolerance in meters is used; the snap/split distance is then independent from any zooming. The default snap tolerance (in meters) is calculated as: the largest coordinate range, i.e. Xmax-Xmin or Ymax-Ymin as defined by the map's boundaries, divided by 150.
When working with the mouse, a snap tolerance in screen pixels is used; the snap/split distance then varies with zooming. The default snap tolerance (in pixels) is 5.