(Control-8) Use the Fixed Radius specified on the Circle Radius control displayed on the Status Bar. Drag the Circle track into position. 
(Control-9) Drag from the edge of the Circle track to the Center. 
(Control-0) Drag from the Center of the Circle track to the edge. The button to the left of the Circle track button selects which method will be invoked by the Toolbar button.
If the Snap Grid is enabled then the Circle center will be constrained to the grid.
Circle tracks are useful for fitting curves into corners and other tight places. Also, they are good to establish initial layouts and finding out "what will fit".
You can join to and from circles, after which they become curved tracks.