The clear aperture is the pupil of the system that preserves all specifications defined by the manufacturer. It is always smaller than the diameter of the optic.
Thickness indicates the distance between two surfaces. Larger thickness allows for greater heat dissipation and higher resistance to stress in machining, blocking, and handling, however it increases weight and dispersion.
Surface quality measures surface imperfections and is expressed by two numbers separated by a hyphen (scratch-dig). The first number refers to scratches and the second to digs or pits, where lower numbers indicate higher quality.