The Adjustable timestep option allows you to specify whether or not the solver is allowed to vary the initially specified timestep or not. If you choose yes, you can pose certain limits which the solver shall use to determine the varying timestep:
- Maximum Courant Number: The timestep shall be small enough such that the Courant number does not exceed this value
- Maximum timestep: Upper limit for the varying timestep
If you are unsure always choose Yes for this option since it will save you plenty of simulation runtime.