When an application connects to the infrastructure to subscribe to market data, behind the scenes, the infrastructure component (ADS) will check with DACS and verify the credentials of the user, and only allow login if the user is permissioned, and only allow subscription to the items that user is permissioned for. No explicit permissioning action is required on the part of the application.
However, there are use cases, that require an application to verify permissions for user(s) without explicitly subscribing that user to data via infrastructure. For example, redistributing data to different user or fanning out data to multiple users. For compliance reasons, every instance of data access should be verified. This is where OpenDACS can be utilized, to verify permissions prior to handling over the data.