Kathy Castle wrote: Project management plan is a comprehensive document which involves sub documents for project management. Stakeholder register is a part of Project Management Plan. When you review the project management plan yopu can easily understand who your projects stakeholders are and who can be your projects stakeholders. I recommend you to read this article related with
Project Management Plan
To be precise, the stakeholder register is part of the Project Documents, rather than Project Management Plan
The full list of what the Project Management Plan and Project Documents include can be found in pg 89 of PMBOK 6