When you make a new selection, previous selections are not replaced. The selection now consists of all previous selections plus the new one. Control Subtract from the selection The new selection area is subtracted from the previous selection area. If the new selection area does not overlap in any way with the previous selection area, there is no change in the selection. If there is no intersection, all selections are removed.
Some modifier buttons have further effects for certain selection types.
These effects are noted under the affected selection type. During the interim, the selection is highlighted by drawing the appropriate shape in contrasting colors. Using The Tools Select rectangular regions Pressing the Select Mouse Button anchors one corner of the rectangle. The Shift key causes the selection area to be a square. In this case the cursor always indicates some point on an edge of the square, but not necessarily a corner point.