Mouse interaction changes when you position the window pointer over the pick flag.   Here is the complete list.

left-click = move the pick flag left or right
right-click = brings up secondary menu with three effects:

window - display another window with just this trace
magnify - brings up new window with magnified time scale
delete - remove this pick

middle-click = Brings up menu to "name" phase (list can be changed if desired)

With Shift Key held down
 

left-click = sets uncertainty of pick
right and middle buttons ignore shift (i.e. same effect as above).

left-click = Use to measure amplitude and period of a signal