Accounting Periods in GB

I note that Accounting Periods can be given a Period No. in the GB specific Accounting Periods table. Where are these used/recorded? I have tried using P1…P3, but when the system converts this to the periods it seems to use the system calendar despite what is said on the Calendar Setup.

On several views and I believe there also is a setting for this, to choose between seeing the information based upon the system calendar or the accounting period, trying switching the views and seeing the affect.