AFModles are higher level models connecting the elements together, making hierarchies, and performing analyses. Elements, on the other hand, represent a specific device or physical equipment. Here is an excerpt from the PI AF User Guide. For more detailed explanation please refer to the user guide and search for Model or AFModel:
"At the core of the Modeling Framework is an architecture that allows you to create a logical model of your process. Models, represented by the PI-AF AFModel
object, are composed of connected elements (AFElement, AFConnection). These elements can represent physical entities in your process, such as tanks, pipes, and process units, or logical entities, such as recipes and summary data. A model provides a way of connecting model elements with directed connections.
Elements in a model are connected through any number of ports (AFPort), which are defined by the element template (AFElementTemplate). A port can be defined as an input port, an output port, or as an undirected port. The port defines how many connections can be made and the types of elements that can be connected."
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Ahmad has pretty much explained what AFModel is and what it is meant for. From the object model point of view, a AFModel extends AFElement to support features like connections and layers. Hence you can setup connection between elements is to allow you to model a process or process flow.
This is not talked about a lot as there isn't a lot of tools that leverage AFModel. One of the tools that uses AFModel is Sigmafine.
In short, AFModel and AFElement have different purpose and there isn't really any direct benefit to use AFModel over AFElement if you are only map your asset hierarchy.