As described in Equations You Can Solve Using Recommended Functions, Partial Differential Equation Toolbox™ can
solve systems of PDEs. This means you can have N coupled
PDEs, with coupled boundary conditions. The solver
solvepde can solve systems of PDEs with any
number N of components.
Scalar PDEs are those with N = 1, meaning just one PDE. Systems of PDEs generally means N > 1. The documentation sometimes refers to systems as multidimensional PDEs or as PDEs with vector solution u.
In all cases, PDE systems have a single geometry and mesh. It is only N, the number of equations, that can vary.