Comprehensive and Detailed Explanation (concise, policy-compliant):

Scrum defines four events within the Sprint: Sprint Planning, Daily Scrum, Sprint Review, and Sprint Retrospective. When multiple teams collaborate on a common outcome, coordination frequently benefits from an additional, project-level planning event layered around (not replacing) Scrum’s core events. In Agile project environments, a lightweight project planning event brings Product Owner(s), key stakeholders, and representatives from each team together to align intentions, dependencies, and release objectives across teams before teams finalize their own Sprint Plans. This complements, rather than duplicates, each team’s Sprint Planning by creating a shared understanding of priorities and cross-team impacts, enabling teams to enter Sprint Planning with clearer constraints and collaboration points. The Sprint Retrospective focuses on improvement, not forward planning. Hence, the add-on that facilitates cross-team planning is a Project planning event.