Im a IBC memeber so I just looked up the show standards PLAKAT: Traditional Plakats
The traditional plakat is a stout, heavy bodied aggressive fish
with distinctive finnage.
Dorsal - The dorsal fin may be either uniformly rounded or
come to point
shortfin HM :Dorsal fin: -, The dorsal fin may be of any shape. A larger, better matching to anal fin. is preferred over a smaller one, provided that the size does not distract from the symmetry of the fish. Like the anal a slightly longer (1/16”) fin is tolerated. The desired effect is typically achieved by an increase in the number of fin rays.
HM -Singletail Dorsal: A variety of shapes are acceptable – semi-circle, quarter circle, rectangular - as long as breadth and volume are displayed. Triangular shape is unacceptable. As with the other fins, width and fullness are important, with maximum fin area a goal. Ideally, the dorsal fin will overlap the caudal fin and appear blended with it, though not physically fused. The first rays (closer to the head) must be comparable in length to the other rays,
Doubletail Dorsal: The base of the dorsal fin of a doubletail Betta is expected to be considerably broader than that found in the singletail. The doubletail dorsal is, ideally, the mirror image of the anal fin in keeping with the concept of symmetry.