RVOverlay nodes can be used to place text and rectangles over any input source. They can also be used to create custom mattes for sources. You can see information about the RVOverlay properties in the Reference Documentation.
Attached are two example session files that make use of the RVOverlay node. In overlay_colors.rv you will find an example of drawing various colored rectangles along with some text annotations. In overlay_coords.rv you will see an example of how RVOverlays use "Normalized Coordinates" to place the paint components.
RV works resolution agnostic. We call the image coordinate system Normalized Coordinates. The center of the image is at 0, 0. The float values that make up the horizontal and vertical positions are measured in units of the height of the image. So are the width and height size values of RVOverlay rectangles.
In order to place something on the left hand side, you will have to set the position of the overlay to:
-1 * ( source width / source height ) / 2
Note that the divide by 2 above is for each half of the coordinate system.
The right hand side would be:
( source width / source height ) / 2 - width of your rectangle
Again, you can see examples of this in the attached session files.