A perpendicular move allows the marker to
move only in a perpendicular direction so that additional line
profiles are made parallel to the original one. To select this,
check the Move Perpendicular command in the drop menu of
the Line Profile button 
on the
main Image Plot Toolbar.
An unconstrained move allows the marker
to move anywhere on the image. To select this, un-check the
Move Perpendicular command in the drop
menu of the Line Profile button on the main Image Plot Toolbar.