A scalar PDE is one of the following:
In all cases, the coefficients d, c, a, and f can be functions of position (x and y) and the subdomain index. For all cases except eigenvalue, the coefficients can also depend on the solution u and its gradient. And for parabolic and hyperbolic equations, the coefficients can also depend on time.
The question is how to represent the coefficients for the toolbox.
There are three ways of representing each coefficient. You can use different ways for different coefficients.
For an example incorporating each way to represent coefficients, see Scalar PDE Functional Form and Calling Syntax.