ARM - Carry forward of balance explanation

Hi,

Could someone provide me with insight as to how the carry forward of balance explanation from one month to the other works?  I thought that you had to manually select prior month rec and carry it forward to current month if you wanted to use the balance explanation again.  I have come across some instances where my balance explanation from prior month has been carried forward automatically by ARM.  In a specific case, the balance explanation and attachments were carried forward automatically upon opening the new period and bacuse the source balance had not changed compared to prior month, it