How did u know that BADI _SLS_HEAD_SCR_CUS is going to be used ? When you went to VA01, we didnt see that BADI there , so how will I know from where the above BADI came from ?
Create only with z ot y as best practice, in case of any upgrade by chance if same field introduce with any append strcture it may give error, So better use y or z.. send a screen shot at my email id zafar.karnalkar@gmail.com
These are BADI not BAPI, we have to check these in SE18 there we will get all information about the selected BADI by there signature. And we can choose according our requirement.