Hi,
One thing you should understand is, this is in planning group. ie You define as to how you will plan to do and how you plan to execute, monitor/control etc.
As per PMI, you should plan everything before you actually do it. Some plans are specifically mentioned within Process Area (like HR, Comm etc) but many are already done as part of PMP in integration. But you should not look at these plans as separate or done sequentially in exactly this order.
Requirements management plan mentions how you will manage the collected requirements, control etc. It does not mention about how you will collect requirements and hence it is output.