This depiction is of a bathroom vanity wall as seen from the side. It includes a vanity cabinet, countertop, and three wall sconces. When your electrician wires your wall sconces he will ask you at what height you would like to see the finished product when all is said and done. In this picture one sconce is mounted on the wall and the other two seem to be floating in mid-air. These other two sconces are shown to indicate where their top, bottom and center will be if installed where the mounted sconce is centered.
Some care must be taken when the decision regarding the placement (height) of wall sconces is being made if the type of sconce has not been decided yet. The electrician will install a lighting outlet box in the wall directly behind the center of the fixture as indicated by the dotted line. This box is not moveable after the drywall is complete and all painting etc is done. As you can see, if this lighting outlet box is installed where the sconce is shown as installed, it may not work if the customer decides to go with another type of sconce ie: one with a globe that points up. It is best to decide on the sconce you want to use, order it, and then with the help of a friend literally hold it up to the wall, moving it up and down until you get to the right spot, and then mark that height on the wall. This way you are much more likely to appreciate the outcome, rather than leaving this decision up to your electrician.