What is the special feature of the Symbols bar?

You can only remove object symbols that are not used in the model. If you remove object symbols that are used in the model, the Symbols bar will be split at the end, and the object symbols will be placed below the separator bar with the note Model-specific. You can use these object symbols to place additional objects. Object symbols that are placed below the separator bar are automatically removed from the Symbols bar when the last object with this symbol is deleted from the model.

The note model-specific is therefore inserted because the object symbols in question are only offered by the Symbols bar for this model. When you create a new model of this type, the object symbols are no longer contained in the Symbols bar. If you want to reuse removed object symbols in a new model, you must add them first.