Hi Hazem,
You can think of it as the project management plan is a always a "work in progress" during the planning phase. for example, when you are doing the scope management plan, you finalize the scope management plan, and then add it to the project management plan. the updated project management plan will then be an input to your next planning exercise which will add/update another subsidiary plan.
this case would be the same for some other documents, for example, the WBS and the WBS dictionary, these documents will be created in the create WBS process and will then be both an input and output to other processes which will keep updating and adding details to the documents.