How to define different equations in different subdomains?

In Custom Equation mode, I don't seem to be able to have a variable take on different values in different subdomains, or alternatively define different equations in different subdomains.

In the define equation mode this seems to be a part of the equation mode settings, but I'm defining my own equation and the option just isn't there.

It is currenly not possible to use different equtions in different subdomains with the built-in FEATool solver, however you can technically manually edit converted/exported FEniCS Python scrips to solve different equations in different subdomains.

You can also define different values for coefficient in different subdomains, either by entering specific numerical values in the coefficient edit fields (on a per subdomain basis). Or use the "Model  Constants and Expressions" with different values separated by one or more spaces, for example and expression like "10  20  30" would result in the coefficient values 10, 20, and 30 for subdomains 1, 2, and 3, respectively. These coefficients and expressions can then be used to turn equation terms on and off (with for example logical switch functions (a > 0)). See for example the Spherical Capacitor tutorial how to define different coefficient expressions in different subdomains: https://featool.com/doc/Electromagnetics_01_spherical_capacitor1