Regret for inconvenience, if my previous question didn't express my query properly..
Well, I have that bad habit of not understanding the question properly.
"dif between deleting a dataset and uncatalog a dataset"
UNCATLG of a dataset means it would remain in the storage area, but the Entry pointing to this dataset in system catalog would be removed. So the next time if you want to acces it again, you may need to know the exact Volume and Serial number etc.
DELETE means the dataset will be deleted from the storage area along with Entry removed from System catalog,ofcourse.. So next time when you want to access this dataset, there is no direct way. Only if your SYSTEM ADMIN is taking a HSM Back-up of the datasets, you can re-access it.