Using different equations in different subdomains?

Jiao Zhao wrote

Different general diffusion-convection-reaction equations cannot be used for different domains such as top and bottom domain shown in Figure 1?  For example, if we want to set up a reaction in the bottom but this reaction is not available in the top domain, We found this setup did not work. Both bottom and top domain will use the same general equation.

FEATool Multiphysics fully supports different equation coefficients for different subdomains (and activating/deactivating equations in different subdmains, and coupling different subdomains). See for example the model example linked below:

https://www.featool.com/model-showcase/02_heat_transfer_03_heat_transfer3

However, FEATool does not support completely different PDE equations or differing FEM discretization schemes (FEM shape/basis functions).

In your submitted image/case you can technically have different reaction coefficients Ri in different subdomains. However, note that in your model you only have a single *1* subdomain.