fontmath.cfg can contain declarations relating to the use
of fonts in math mode.
If it exists, it defines which fonts in which sizes are used in math mode, and how they are used. It also defines all the math mode commands that `are likely to' depend on the choice of math fonts used (e.g. commands that depend on the position of a glyph in a math font).
The main reason for the existence of this file is to provide for future updates when a standard math font encoding is available. Right now we do not encourage the use of this configuration file other than for special applications. Writing a proper configuration file for math mode needs expert knowledge!
Please note that use of this configuration file has the following consequences.