The BC means the base arc of the pupil. The base arc is the curvature of the pupil, which is routinely used around 8.6mm. On top of the 8.6mm, fluctuations of 0.1mm up and down are suitable for Chinese people. The base arc for regular use is 0.2mm higher than the normal base arc of the cornea, so that there is no significant pressure on the cornea or the possibility of contact with the cornea, which makes it more appropriate to wear. The larger the diameter, the better the pupil will look, because the larger the diameter, the wider the coverage on the lid cornea. Therefore, the BC and DIA represent the base arc and the diameter.