The classification of airfoils as flat bottomed or semisymmetrical is a very crude way of classifying airfoils. The lift characteristics of an airfoil are closely related to the mean camber line and the drag characteristics are related to the thickness. A thin, flat bottomed airfoil can have less mean line camber than a thick semisymmetrical airfoil making it difficult to draw conclusions about the way the airfoil behaves from such a simple classification.

Over simplification of lift theory led to serious misunderstanding by a lot of people. Airfoil classification simply as flat bottomed or semisymmetrical is an over simplification that can lead to serious misunderstandings too.

As a case in point, a flat plate is a flat bottomed airfoil. The fact that it is also symmetrical tells us that it has no camber of the mean line and the fact that it is flat on both sides implies that it has little thickness but doesn't tell how thick so we don't know much about the drag. The lack of camber tells us that it will perform the same upright as inverted. So, it takes a lot more information than just flat bottomed or semisymmetrical to begin to draw conclusions about how an airfoil will behave.

In the 1930's NACA adopted a four digit airfoil designation system based on the camber of the airfoil's mean line and the airfoil's thickness because these numbers had primary aerodynamic significance. Later, NACA refined the designation system by increasing the number of digits to five so that the location of the high point of the mean camber line could also be included. As laminar flow airfoils were developed in the early 1940's NACA increased the number of identifying digits to six, one of which was a subscript, so that the distribution of thickness along the chord could also be identified by the designation.