query in pdepe function

I've a doubt in using the pdepe function which solves initial-boundary value problems for parabolic-elliptic PDEs in 1-D.

In the theory, 3 terms are described. c,f and s. f is a flux term, s is source term and c is said to be The coupling of the partial derivatives with respect to time is restricted to multiplication by a diagonal matrix "c".

If anybody is familiar with this, can you please tell me what exactly the term "c" stands for?