Complex GoldSim models may have hundreds (or even thousands) of elements. In order to manage, organize and view such a model, it is very useful (in fact, essential) to group the elements into subsystems. A subsystem is simply a collection of elements.
Subsystems are created in GoldSim by placing elements into Containers. A Container is simply an element into which other elements have been placed. In a sense, it is like a "folder" or a "box".
Containers can be placed inside other Containers, and any number of layers of containment can be created. This ability to organize model elements into a hierarchy creates a powerful tool for creating "top-down models", in which the level of detail increases as you "drill down" into the model. You can even "lock" a Container (with a password), which allows other people to view and use its contents, but prevents them from modifying the contents in any way.
The ability to create subsystems using Containers (and protect their contents) provides a powerful capability: the reuse of subsystems. A user can create a complex subsystem, and then document and save it, such that a subsequent user could simply drop the subsystem into a new model. This facilitates the creation of a library of documented and verified subsystems. Such a library can be used within an organization to quickly and efficiently build complex models.