A placement ID is a unique identifier, automatically attributed to each ad placement you create using the Appnext monetization solutions.

You can find the placement IDs clicking the "Settings & Placements" button under the "Manage Apps" tab.

It is important to create a separate Placement ID for each ad placement you plan to activate, within your apps or websites (App Launch, In-Feed Unit, Content Widget, etc.). It will allow you to accurately collect data across placements, better understand unit performance and optimize accordingly.