The Tool Options Panel allows you to specify:
Name : Used for the new layer; any number in this field will automatically increment with each layer created
Blend Method Parameter : The blending method used to composite on the layers below
Opacity : Sets the Amount Parameter for new layers. Defaults to 1.00 (Completely opaque)
Spline Points : Specify the number of Spline Points that will be used in the new Splines (for other than Circle Layer)
Offset : Offset of the tangent of the Spline Points (for other than Circle Layer)
Invert : Whether invert the create(d) layer(s). Is related to his Invert.
Falloff : Determines the falloff function for the feather (only for Circle Layer)
Link Origins : Whether Link the Offset of Splines
Spline Origins at Center : Set the origin of the layer at the center of the circle, else the center is set at the Canvas center
Click on the canvas where you want to have the center of the circle and drag to obtain the desired radius value, and release the mouse button when you are done.