When using fillet you can draw an arc between two linear entities, let's say two lines. The new arc is exactly tangent to the two lines used for filleting. For using fillet you need two linear entities. With the 'Tangent Arced Segment' command you can add arced segments to polylines that are tangent to the end you select. After selecting the polylines the segment will appear at the nearest endpoint. The segment will follow the cursor until you pick a point. At that moment the segment will be fixed. It's possible to fixate the radius of the segment. In that case the movement of the cursor will determine the length of the segment. You can also set a specific radius. The command will make a suggestion of the radius based on the radius of the arc at that moment. After determining the radius the cursor will define the length of the segment again. The new segment will always be exactly tangent to the side of the selected polyline allowing you to create perfectly flowing polylines.