The contract with a provider that processes personal data on your behalf.
Whether a data processing agreement exists is something you check with the provider yourself and enter in the data inventory: yes, no or unknown. Unknown is an honest answer and better than a guess. Legal questions are for an adviser.
Example from Module 2
For one tool you do not know whether a data processing agreement exists: you enter unknown and check.
Module 2: “Protecting data: encapsulate and check”. In the Werkbank course room, with sign-in.