Item charge permissions issue

Hi All,

We have received a issue from client, wherein they require to use the Item Charge functionality in NAV 2015.

For that they need to create the new Item Charge. They don't have the permissions to do in NAV.

We have assigned them the require permission to Read &Insert into table data 5800.

But when they tried to create a Item charge with "Gen. Prod. Posting Group.", they get error message saying that "You do not have
permissions to Modify the Table data 5800, Item Charge.

I saw the code for the Onvalidate trigger of field "Gen Prod Posting Group" and found below:

IF xRec."Gen. Prod. Posting Group" <> "Gen. Prod. Posting Group" THEN
  IF GenProdPostingGrp.ValidateVatProdPostingGroup(GenProdPostingGrp,"Gen. Prod. Posting Group") THEN
    VALIDATE("VAT Prod. Posting Group",GenProdPostingGrp."Def. VAT Prod. Posting Group");

Could anyone help me what exactly it doing ?

Thanks in advance!!