A custom table is a rectangle where you can set the number of seats on each of the 4 sides independently. It can also have curved table ends.
Add a custom table using Insert>Custom Table... .
To add round ends set End 1 curve and End 2 curve to the appropriate degree of curvature (0 degrees=no curve, 180 degrees=maximum curve).
If auto is checked the table size will be automatically adjusted to the number and size of seats.