If you are using delivery data control then it will look at purchase orders and the expected delivery date of the orders as being available inventory. This is a quick walk through of the effect of the purchase order on ATP.
You will need to factor this into your planning and decide how reliable your supply chain is for items. For example if supplies are often missing their delivery date and you had promised to a customer stock then you are going to annoy your customer if the supplier doesn’t come in on time. The purchasing team will need to keep orders current on delivery date so you can leverage this capability.