When a well-balanced round diamond is observed face-up, it owns a perfect round shape and its table is a regular octagon.
In a regular octagon, all eight sides are of equal length and all interior angles have the same value of 135°. Plus, each of the opposite sides is parallel to each other and will never intersect.
If a diamond has a deviation in its table shape, we call it non-octagonal table (T/oct). In comparison to a regular shape, the values of side lengths are not all equal in a non-octagonal table facet, nor are the interior angles.
Since the table facet and its adjacent star and bezel facets interplay, diamonds with non-octagonal table usually contain deviations in these facets as well, including misshapen facets (e.g., misshapen stars and misshapen bezels), star angle variation (SV), star percentage variation (SPV), or table off-center (T/oc).
It should be emphasized that the table is the largest facet of a round diamond. Similar to the case of table off-center (T/oc) deviation, non-octagonal table is also relatively easier to get caught, even by naked eye without the aid of diamond loupes or microscopes for magnification.
As a diamond's symmetry grade is basically determined when face up (though sometimes face-down inspection may be needed for confirmation), any table-related deviations produce a big impact on the assignment of the final grade.
Finally, the undesirable geometric appearance and the visually inconsistent light patterns and uneven brilliance, fire and sparkle of a T/oct diamond would cause a low cut grade.
However, as long as the geometric and visual variations aren't prominently unbalanced when viewed with naked eye, the stone is still worth considering.