suppose ur GU call is getting a segment 3 level down the hierarchy. Now you are giving 3 qualified SSAs for it. One for the root, second for the parent of the segment being fetched and third for the segment being fetched. If you use a plain GU without command codes, you get only the 3rd level segment in your IO Area and the parentage is set at the 3rd level. But if you want to establish parentage at the root level or at the 2nd level, then you can use a command code (*DP after segment name) in the corresponding SSA, and you will get the 3rd level segment along with the one or two extra levels that you want. Also the parentage will be established at the level whose SSA has the P command code. Make sure not to use the P command code in more than one SSAs. You can use the D command code in both upper levels though.