The high-level scale is from A to D, and each grade above D is further split into three (C, CC, CCC etc.) with an additional +/- modifier at each step except AA

The high-level scale is from A to D, and each grade above D is further split into three (C, CC, CCC etc.) with an additional +/- modifier at each step except AAA. That makes 25 grades above D if I’m not mistaken.