Solvent B is usually a 0.02 - 0.1 M solution of buffer containing the upper limit salt concentration for your gradient. In this simulation, you need to choose the salt concentration in both Solvent A and Solvent B reservoirs.
This simulation is designed to have a specific mobile phase composition profile. The mobile phase starts out as 100% Solvent A during the loading of the sample and an initial wash, followed by a linear gradient from 0% Solvent B to 100% Solvent B.
In modern chromatography systems, it is usually possible to use the mixing valve to select any ratio of Solvent A to Solvent B throughout a chromatographic run. In such systems, Solvent A is usually 0.02 - 0.1 M buffer (no salt) and Solvent B is usually 0.02 - 0.1 M buffer containing 1 M NaCl or KCl. The user can then select to do a simple gradient (0 - 100% B over 30 min, which would give 0 - 1 M NaCl over 30 min) or a more complicated gradient (20% B for 5 min, then 20 - 50% B over 25 min, which would give 0.2 M NaCl for 5 min, followed by a 0.2 - 0.5 M NaCl over 25 min).