I'm new to the Project Management world, but I hope I can help you with this piece of information from Andy Crowe's book (5th edition, pp. 86, 87):
_(About Scope Management Plan, but it applies to all other processes and I believe it answers your question)_
> "This process is a bit unusual in that it appears to be part of a loop. [...] Scope Management Plan is part of the Project Management Plan, which is an input into Plan Scope Management. To understand this, consider that this process has several parts that are developed together. **The unfinished Project Management Plan is brought in, and the Scope Management Plan is produced.** The Project Management Plan, although unfinished at this point, is brought in to help determine how the scope will be gathered, defined, broken down, validated and managed.