Focus on products principle
In PRINCE2, even before the project begins, there is a need to understand and define what must be delivered from the project in order to ensure the benefits are realized. This is done by writing a project product description. This is written by the project manager in consultation with users. Its purpose is to understand the features or characteristics of the project product (i.e. the one handed over to users at the end of the project) which will make it acceptable to the customer.
Early focus on the scope
By developing and agreeing the project product early on, it focuses attention on the scope of the project and be useful when developing initial estimations for the time and cost required to build the product.
When planning each subsequent stage, more detailed product descriptions are developed for the major products of the project and are incorporated into the appropriate stage plan. This means that each stage plan contains the details of what will be delivered and the job of the project manager is to ensure these products are delivered at the right level of quality within the agreed constraints of cost and time.
So, when planning either the project or a stage, attention is focused on what will be delivered before a decision is taken to proceed to the next stage and spend any more money.