To calculate flow field it is required to solve the Navier-Stokes equations inside the flow domain. This is a partial differential equation system which cannot be in general solved analytically and needs to be converted into a linear equation system. The process of converting partial differential equation into a linear equation system is called discretization and can use multiple different schemes.
In the Discretization panel you can select discretization schemes that are most suitable for your simulation. Usually, when selecting discretization scheme there is a trade-off between accuracy and stability, which may require some experimentation to select optimal scheme.