Turnout object is a feature. Feature has only Feature Definition and feature name properties.
Additional properties you can see in the dialog are coming from the turnout feature definition.
Therefore changing any of the properties, like Description, will change all the turnouts that are using the same feature definition.
If you want to add per turnout information, you can attach Item Type to the turnout feature definition.
Video below demonstrates how to proceed:
- Create Item type and get properties from the turnout
- Add Item Type to turnout feature definition
- Annotate turnout using Item Type
Note that Item Type and adding Item Type to the feature definition should be done in relevant DGN libraries (dgnlib), not in design file.