Sir if we maintain 3 workdays or more here in special rules in Factory calendar then based on that billing docs will be generated as you said that is fine but my question is Does it has any impact while doing Delivery scheduling to propose delivery date because system considers factory calender while proposing delivery date
Delivery date in sales order are linked to the factory calendar assigned to the shipping point. Different factory calendars are created to shipping point and billing, so they are not linked.