One of the outputs of the project schedule is a detailed plan. What is the main purpose of that detailed plan?

The project schedule provides a model of when project activities will be performed and when deliverables will be produced, directly linking planned work to the scope baseline. It represents the time-phased plan showing how the scope will be executed, allowing the team and stakeholders to understand the expected delivery sequence and timing of all project outputs.

Creating a formal record of the project and showing organizational commitment describes the purpose of the project charter, not the project schedule.

Describing how scope will be defined, developed, monitored, controlled, and validated is the purpose of the scope management plan, not the project schedule.

Providing the needs of a stakeholder or stakeholder group describes the purpose of requirements documentation, which is an output of the Collect Requirements process.