I want to understand the relation of the Planning Period Filter for months relation to the INV ordering module and how the data changes
Looks good but with maybe a small tweaks. You'll use a saved view from your SYS00 Time Settings by Month module using the filter that only shows months. That will allow you to filter on the calendar, for instance, you can filter the months in the H1 FY20 Half. Then you can check the months you want to plan and the weeks in your planning grid that are in the months you checked. You can use a custom view on the planning grid and use the SYS01 Time Settings by Week filter "Planning Period Filter" set to true.
Ok got it thanks @JaredDolich !