In the D-Wave system, a solver is simply a resource that runs a problem. Some solvers interface to the D-Wave QPU; others leverage CPU and GPU resources to solve problems. Software solvers, like optimizing and sampling emulators, mimic the problem-solving behaviors of the QPU and are useful for prototyping algorithms that make multiple calls to the hardware.
Before you submit a problem to the D-Wave system, you need to know the properties of the solver and what parameters it accepts with the problem submission—how many reads of the problem, you want, for example.
If you are using DWaveSampler, use the following command to return its properties: