An oval floor in Minecraft needs a width and a depth. One radius cannot describe it. The ellipse here is the midpoint ellipse: horizontal radius along X, vertical radius along Z on the grid. An example of 8 by 5 is a low, wide ring, the sort of footprint people use for a hall that should not be a perfect circle. Thickness adds inner ellipses the same way a thick circle adds inner circles, so a wall can be two blocks without becoming a filled rug.
Row counts are more useful on an oval than on a circle, because the rows are not a mirror of a single radius in both directions. The long middle row is the horizontal diameter. The short ends shrink faster than a circle would. If you are placing blocks and the end caps run out of blocks earlier than you expected, that is the ellipse, not a missing row. Compare your placed count with the table before you "correct" it back into a circle.
Numbers follow the angle around the outline, same as on the circle tool. They are labels. The PNG is the current grid. A sphere cannot be made by stacking these ovals automatically; a round ball is on the sphere page, and a round floor is on the circle page.
Very flat ellipses, such as 20 by 2, turn into a long bar with pointed ends. The maths is still an ellipse. If the shape you wanted was a stadium (a rectangle with semicircle caps), this is the wrong outline. A stadium is not an ellipse, and forcing the radii will not produce straight sides.
Both radii are block counts from the centre block to the edge, in the same sense as the circle radius. The centre cell is included in the coordinate system even when the outline does not fill it. On a thin oval the centre is often empty, which is what you want for a room.
Decide the two radii from the room, not from a circle you then squash by eye. If the hall should be 17 blocks wide and 11 blocks deep, the horizontal radius is 8 and the vertical radius is 5, because each radius is the distance from the centre block to the edge and the centre itself accounts for the odd block. Typing 17 and 11 would draw something far larger than the foundation you marked. Thickness is the wall. An 8 by 5 oval at thickness 1 is a thin ring. Thickness 2 pulls in a smaller oval and gives you a rim you can walk. Set thickness to the smaller radius only when you want the oval filled.
The row table is the build order. Place the long middle row first so the width is obviously right, then the rows above and below it, matching each count. The ends are short. That sudden drop is the ellipse tightening, and "correcting" it into a gentle circle produces a circle, which you could have drawn on the circle page with one radius. Save a PNG of the finished grid if you are building across more than one session. The picture matches the numbers only for the radii on screen at the time you download it.