Quantity of sales order in Milestone wise billing

Hi

i have a sales order and its assigned to Project. Its a project sales order and we are doing milestone billing.

only one line item in sales order. Now my question is we dont give quantity in the sales order line item and now the slaes order is saved and the quantity filed is grayed out.

Why is it happening. Any speciff setting we mainitain for Project related sales order for milestone billing plan???

Please guide me

Thanks a lot in advance

Regards

SURYA