Many scientific databases can be decomposed into subsets that represent only part of the database. It is common to decompose databases into subsets based on Materials (regions), Domains, Groups, AMR patches, AMR levels, Species, Assemblies, etc. Databases are decomposed for a variety of reasons including breaking the database into smaller sized pieces that can be calculated in parallel using a computer with multiple processors. When VisIt is used to visualize the results of a simulation that has been divided into smaller subsets, it is often useful to restrict the size of the database, to increase performance, by using VisIt's Subset Window.