Linking models is primarily intended for linking separate buildings, such as those that compose a campus.
You can link architectural models, structural models, and MEP models.
For example, the following site plan shows 4 building models linked to one model.
When you link a model into a project, Revit opens the linked model and keeps it in memory. The more links a project contains, the longer it can take to open.
Linked models are listed in the Revit Links branch of the Project Browser.
You can convert linked Revit models to groups, and you can convert groups to linked Revit models. See About Converting Groups and Linked Models. You can also mirror linked Revit models. See Mirror Elements.