VASSAL Reference Manual
Folders can be created under many components to group together similar sub-components. For example, you can create Folders under the Global Properties component to organise sets of Properties, or under the Prototype Definitions to organise your prototypes.
Folders can be nested to any depth and are transparent to Vassal. Whatever is stored in a Folder, is seen as stored under the top-level component that contains the nest of Folders.
Configuring a Folder