Item Details Columns of Production Order

Hi,

In a Production Order Component overview screen,i can see 4 columns related to item details- ITEM, OpAc,ItemID,ItemNo.

Can anyone explain what is the use of above 4 columns and how are they different from each other. Does maintaining all the above 4 columns are mandatory. Can we make changes to the Column-OpAc,Item,ItemID,ItemNo once a production order is created and released .

Thanks in advance.