The Inscribe/Circumscribe Circle tool either inscribes a circle in a triangle or circumscribes a circle about a triangle. This tool can only be applied to triangles. You toggle between inscribed and circumscribed using the second Option Button. There is one thing you should note though about the effect of Paint Attributes though. If the triangle you’re inscribing/circumscribing has a thick stroke width, the circle may overlap the corners or interior of the triangle. Circumscribing/inscribing is based on the dimensionless points that defined the corners of the triangle. It doesn’t account for paint width. The best way to adjust for this is to change either the paint width of the circle or the paint width of the triangle. You might also try Scaling the circle after the construction. Also, note that if your circle uses a drawing style of “FILL” or “FILL and STROKE” then the circle will obscure the triangle. In most cases, you probably want to use a drawing style of STROKE. The only other construction option is the number of Snapping Points. These are the invisible along the circle to which you can snap other paths. For more information, see the explanation of snapping points in the Circle tool.