When creating the project product description, how should the purpose of the project product be defined?

The purpose of the project product, as defined in the Project Product Description, is to enable the achievement of one or more business outcomes. An outcome is the result of the change that the project's product (or output) brings about. These outcomes are what lead to the realization of benefits detailed in the Business Case. Therefore, the product's purpose is directly linked to the change it will create for the business, which is best described as an outcome. This ensures a clear line of sight from the project's deliverable to the value it is expected to generate.

A. An opportunity is the rationale for undertaking the project, which is documented in the Business Case, not the purpose of the product itself.
B. A vision is a high-level, aspirational statement for the project or product, which is broader than the specific purpose defined in the Project Product Description.
D. A goal is a high-level target. The project product is a means to achieve a goal, but its direct purpose is to enable the outcome.

AXELOS. (2015). PRINCE2 Agile® Guidance. The Stationery Office (TSO). Section 9.3.1, "Outputs, outcomes and benefits," p. 86, clarifies the relationship: "A project delivers outputs... which are then used to bring about change (the outcomes), which in turn create the benefits."

AXELOS. (2015). PRINCE2 Agile® Guidance. The Stationery Office (TSO). Section 12.2.18, "Project Product Description," p. 132, notes that its purpose is to define what the project must deliver to be accepted. This acceptance is based on its ability to enable the desired outcomes.