The Data Model refers to the collection of primitive types and abstract concepts within the FrooxEngine, and are therefore synced with the model when defined within the code base in the engine.
An example of a type that is not defined currently (or at least not very supported) with the Data Model is the GUID type, and thus has a high potential of breaking the engine, causing a crash when used in certain ways, and many other oddities.
Local elements are also not part of the data model and thus also not synced, but are tracked using the Is Local Element node. There is currently no way of making a slot have this kind of behavior without the use of mods.
for a complete list of what is in the data model, see core concepts.