Hi Venkata,
SPDD are the data dictionary modification and SPAU are the object modification which occurs when you perform either patch upgrade or version upgrade for the objects for which you have made any modification in your source system while normal production operation to fix some issue and the changes to the same objects are provided by the version upgrade. SAP provides you an option to either use your changes or use the changes provided by the upgrade for those objects.
SPDD modification (use version from the upgraded patch or use your original modification) needs to be decided during the upgrade phase whereas for SPAU, SAP provides 14 days to make the decision. If the decision for SPAU is not taked within 14 days changes are reset to original.
Let me know what upgrade you are doing and what strategy are you using to provide specific details.
Thanks,
Manas