Hold the Ctrl key down, click the separating line between the tracks and draw up a marker with the mouse key pressed. Holding down the Ctrl key also allows you to adjust beginning and end of the marking with the mouse or to move the marker altogether.
Or you can right-click the separating line and select Set left marker or Set right marker to define the required area.
To remove the marker area, right-click the marker and select Remove marker.
You can protect the track marker from unintentional editing by right-clicking it and selecting Lock marker. The same way locking can also be made undone.
Tip: If you want to highlight several
portions in a way which can be reproduced, set a a position marker
at beginning and end of the portion. Using tool 
Snap to objects you can adjust
the marker most accurately. See also Toolbar
and Using
markers.