The UserObject system is developing and running custom algorithms that may not fit well within any other system in MOOSE. Examples include complex calculations that may result values that don't associate in a one to one manner with elements, nodes, or sides. Perhaps your user object produces values based on height in your domain or based on groups of related elements (not necessarily associated with subdomains or other static mesh features). Often these calculations may result in custom data structures that can be managed by the developer.
The UserObject system is the basis for the Postprocessor system. The only difference being that Postprocessors have an additional interface for returning values and corresponding managed storage within the framework for retrieving those values through well-defined interfaces \(PostprocessorInterface\).
One benefit of the UserObject system is that it is designed with user-defined APIs in mind. UserObjects can and should define additional
public methods that may be used to retrieve data computed by the UserObject. The UserObjectInterface is templated so that an object that depends on a specific type of UserObject may retrieve that specific type of UserObject and use the interface without the use of dynamic casts.
Types of UserObjects
GeneralUserObject: "execute" is called once on each execution flag.
NodalUserObject: "execute" is called for each node within the mesh on each execution flag.
ElementalUserObject: "execute" is called for each element within the mesh on each execution flag.
InternalSideUserObject: "execute" is called for each side, that is not on a boundary, within the mesh on each execution flag.
SideUserObject: "execute" is called for each side, that is on a boundary, within the mesh on each execution flag.
Since UserObjects often create and store large data structures, the developer of a UserObject should consider whether or not those data structures needs to be declared as "restartable". Knowing whether or not a data structure should be declared restartable is straight-forward: If the information in a data structure is expected to persist across time steps, it must be declared as restartable. Conversely, if the data in a data structure is recomputed at each invocation of the UserObject, no action is necessary. See restart_recover.md for more information.