A Playbook is a set of best practices that guide sales representatives through the sales process. A Playbook consists of Actions and Activities that are triggered by certain conditions, such as sales phase, lead source, or product category. Actions are tasks that sales representatives must perform, such as updating a field, sending an email, or creating a quote. Activities are events that sales representatives can schedule, such as calls, meetings, or demos.
To require the sales representatives to update the Revenue End Date when Opportunities are in the "Close" sales phase, the best approach is to configure a Playbook with a mandatory Action to update the field. This way, the sales representatives will see a notification on the Opportunity header that reminds them to complete the Action before they can change the sales phase. The Action will also appear in the Playbook pane, where the sales representatives can mark it as done after updating the field.
References = Playbooks, Configuring Playbooks, Viewing and Completing Playbook Actions and Activities