The Single Row Marquee Tool creates one pixel high and all image wide row selections:
To select a row In the Toolbox, select the Single
Row Marquee Tool and click on the image.
While New selection option choosen on the Options bar by creating new selection you delete existing selection.
To add a row to existing selection you have
either to choose Add to selection option on the Options bar or to press (and hold)
[Shift] key while selecting.
To subtract a row from existing selection you
have either to choose Subtract from selection option on the Options bar or to press [Alt]
key while selecting.
Like with other selecting tools you can intersect new row
with existing selection (either by choosing Intersect
with selection option on the Options bar or by pressing [Shift+Alt]
keys while selecting. However the result will be the new selection.
While New selection option is chosen and
the point of cursor is within selected area
the tool looks like this and can move selection (ie. borders of selected area and not image content).
While New selection option is chosen you
can also move selection using keyboard cursor keys.
*In the above movie colors are inverted just to make selected area visible (it turns black). **Selection techniques explained in detail in the 2nd chapter of this video-course