In the diagram, the visual elements are used to represent different objects and their statuses in the parking domain:

1. **Shapes**:
   - **Rectangles**: Represent curbs. Each rectangle corresponds to a curb where cars can be parked.
   - **Circles**: Represent cars. Each circle is a car that can either be parked at a curb or double parked behind another car.

2. **Colors**:
   - **Green**: Indicates a "clear" status. For a car, it means the car is double parked and has no car behind it, thus it is free to move. For a curb, it means the curb is clear of any cars.
   - **Red**: Indicates a "not clear" status. For a car, it means the car is parked at a curb with another car behind it, thus it cannot move. For a curb, it means the curb is occupied by one or more cars.

3. **Sizes**:
   - **Medium Rectangles**: Used for curbs to provide enough space to visually contain one or two cars.
   - **Small Circles**: Used for cars to fit within the curbs and represent both single and double parked cars.

4. **Relative Locations**:
   - Curbs are aligned horizontally in a grid, with each curb positioned sequentially from left to right.
   - Cars are placed within or behind the curbs they are associated with. A single parked car is placed centrally within a curb, while a double parked car is placed behind the parked car within the same curb.

5. **Legend**:
   - Provides a mapping of colors to statuses, helping to interpret the diagram by showing which colors correspond to "clear" or "not clear" statuses.

6. **Text Labels**:
   - Each object, whether a curb or a car, is labeled with its identifier and status. The labels are placed inside their respective shapes to ensure clarity and readability.

The combination of these visual elements allows for a clear and intuitive understanding of the parking configuration, including which cars are free to move and which are not.