In the Opitz classification system, the second digit is used to indicate the external shape and external shape elements of a part or component. This system is designed to categorize and specify the characteristics of geometric features based on their shape and structural elements, which is important in manufacturing and mechanical engineering.
The first digit of the Opitz system denotes the basic classification of the part, while the second digit provides further detail about the shape and external features, which is crucial for defining the manufacturing processes and the tools required.