You can group elements in a project or family and then place that group many times in a project or family. Grouping elements is useful when you need to create entities that represent repeating layouts or are common to many building projects.
With each instance of a group that you place, there is associativity among them. For example, you create a group with a bed, walls, and window and then place multiple instances of the group in your project. If you modify a wall in one group, it changes for all instances of that group, simplifying the modification process.
A group cannot contain both model and view-specific elements. If you select both types of elements and then try to group them, Revit creates a model group and places the detail elements into an attached detail group for that model group. The result is the same if you select both detail elements and a model group; Revit creates an attached detail group of the detail elements for that model group.