These tend to be based on the material composition, for example BK7 is the Schott Glass classification of a common borosilicate crown glass.
The international glass code is based on U.S. military standard MIL-G-174, and is a six-digit number specifying the glass according to its refractive index nd at the Fraunhofer d- (or D3-) line, 589.3 nm, and its Abbe number Vd also taken at that line.
For example, BK7 has nd = 1.5168 and Vd = 64.17, giving a six-digit glass code of 517642.
[1] Consequently, a linear approximation for the refractive index dispersion close that wavelength is given by: where
Note that the glass properties can vary slightly between different manufacturer types.