In the 'planning' technique of PRINCE2, understanding what makes the outputs acceptable to the users occurs during the step of "Defining and Analyzing the Products." This step involves detailed discussions about product requirements and specifications to ensure clarity on what the project must deliver for it to be accepted by the users. It sets out the criteria that the products must meet to satisfy the user's needs and expectations.
References: (PRINCE27 Foundation Handbook, AXELOS)

The expected success level for benefits, time, cost, quality, scope, sustainability, and risk in a project should be set by the project baseline. The baseline establishes the initial scope of what the project is expected to deliver, including standards and targets against which project performance can be measured.References:
PRINCE2 7 Foundation sources on project planning and control.

Section: Volume B