Introduction
The NACA 6-Series airfoil profile is automatically generated from the series designation (63, 64, 65, etc.) indicating the location of minimum pressure, the ideal lift coefficient, and the thickness to chord ratio according to the functions that specify the airfoil shape. The airfoil name will also automatically be altered as these parameters are changed according to convention. Note that you also have control over the mean line x/c = a value which indicates the location along the chord where the chordwise loading changes from uniform to linearly decreasing toward the trailing edge.
The “6A” series aims to alleviate the sharp trailing edge cusp that is present in normal 6-series airfoils. It is important to note that for 6A series airfoils, the upper and lower surfaces are relatively straight from about 0.8c to the trailing edge which implies that a = 0.8 for this class of airfoils by definition. An update in OpenVSP v.3.31.0 fixed a = 0.8 and disabled the parameter for all NACA 6A airfoils.
[Abbot and Doenhoff, “Theory of Wing Sections”, section 6.8, 1959]