The order a payment allocates to a procedure.
FIFO - Fist in First Out is the default allocation order when the user clicks Apply.
You can also select from the transaction screen any unpaid procedures also known as 'line item' since you can select whic procedure line to pay regardless of it's aging.
Unless the user explicitly selects a specific transaction (aka: procedure and/or patient) to which a patient payment is allocated, the system allocates to the oldest transaction with an unallocated patient portion