What is the difference between creating Item structure In PIM and PD?

Please help me to clarify the below points:

1. When do we create item structure in Product Information Management?
2. When do we create item structure in Product Development?
3. What are the major differences when we create item structure in PIM and PD?
4. Do we need to create Specific Class for Engineering items created in PD or Specific Class for Non-Engineering items created in PIM?