User pseudonyms can be resolved by the systems operator

While other users only see user pseudonyms, the system operator knows the real-world identity of all users

While others only see pseudonyms and no further identifying information, the App Operator knows the real-world identity of all App Users.

For example, an e-mail provider may allow their App Users to pick arbitrary e-mail addresses that do not indicate their real-world identity. Anybody can send to and receive e-mail from that e-mail address, without knowing the real-world identity of the corresponding App User.

However, the e-mail provider may require the user to provide a real-world address and credit card to the operator, and is thus fully known to the App Operator.