When a reminders process is run, the "Process as of" date value is one of the things that is set up.  This is ONLY a factor in determining which records will be included in the reminders process.  Any installment that has an installment date that's earlier than or equal to the "Process as of" date that has not been paid will be pulled into the reminders.

Once a process has been run and the reminders letters are created, there is an option to mark the reminders as sent.  Whether or not the reminders have been sent has no bearing on whether an amount mvoes from the CURRENTAMOUNTDUE to the PASTDUEAMOUNT column.  The only think that marking reminders as sent will do is add a date to the letter that's added to the revenue record as a result of running the reminders process.

The PASTDUEAMOUNT and CURRENTAMOUNTDUE values are not calculated based on the "PRocess as of" date.  They are calculated based on whether or not the reminders process has been run and included those installments previously.  The first time an installment is pulled into a reminders process, it will appear in the CURRENTAMOUNTDUE column.  However, in every subsequent run of the process after the first, the installment will be moved to the PASTDUEAMOUNT column, until it is paid off.  Then, it will be removed from the process.

There is a "Clear results" button on the Recent status and History tabs of reminders processes.  This allows users to run the reminders process for the first time to test/check it, and then wipe out that run so that the same installments will stay in the CURRENTAMOUNTDUE and PASTDUEAMOUNT columns if the process is run again.  This can be done repeatedly until the reminders are actually sent.