The ATP time fence as a default lead time if the product you are trying to sell or promise an order date isn’t available. If you do have no inventory for the item then the assumption is that after the end of the ATP time fence you will have inventory. This is a quick walk through of the affect.
When you set the ATP time fence you will have to work out a time that is appropriate for your environment. This many need specialization by product or by product in a site so you have the option of setting this down at the product level to override the default accounts receivable parameters. or setting it down on the site level if for example the sites are spread out a wide distance. When setting this you want to take into consideration the typical lead time of getting the product to give the planning team enough time to get the product in.