One main use of a visualization tool such as VisIt is to compare multiple related simulation databases. Simulations are often run over and over with a variety of different settings or physics models and this results in several versions of a simulation database that all describe essentially the same object or phenomenon. Simulations are also often run using different simulation codes and it is important for a visualization tool to compare the results from both simulations for validation purposes. You can use VisIt to open any number of databases at the same time so you can create plots from different simulation databases in the same window or in separate visualization windows that have been locked together.