Results analysis WBS , creation and usage of reserve

Hi Expert,

In results analysis configuration when we assign line ID to category , we do have secondary cost element for creation and usage.

Conceptually how does it works ?

I do have requirement to post results analysis  reserve every month and from next month on wards it should be revered old RA reserve completely  and should create and post new accrual.

Is any one can explain me the above concept.

Thanks in Advance,

Regards,

Chana