Selector Tool - Transformation Center Grid
The Transformation Center Grid on the Selector Tool Infobar is used to move the transformation center. Clicking on one of the nine buttons moves the transformation center to the equivalent position on the selection. For example, pressing the central button moves the transformation center to the center of the selection.
The same effect can be achieved by CTRL-dragging the transformation center. This constrains the transformation center to one of the corners, the center of the edges or the center of the selection, whichever is closer to the mouse.