Most venues have rounded or arching borders defining their Sections. You can draw this with the designer tool.

The best thing to do when drawing a Section is to create multiple points across the curvature or rounded border you wish to lay down, as seen below. The more points, the softer and smoother the curvature will look.
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If you make a mistake, that's fine! You can always right click to remove the last node you've created. Learn more by looking at the status bar at the bottom of the screen while you are on the Section tool.

If you want, you can hold the alt key while drawing to disable snapping to other points, or deselect Snap to grid for an even higher precision. Don't forget to turn Snap to grid back on when you are done. This will make it easier to lay down rows symmetrically across the seating plan.

If you had already drawn a section but wish its curve was smoother, you can always use the Node tool to click on the sides that look too rough and add additional nodes.

Please note that using a Reference chart to draw on top of is always the most effective and fastest way to draw a precise seating plan. You can upload one by dragging an image to the Reference chart property, visible when no object is selected on the right hand of the screen.