Sales Based Settlement : What is payment previous period?

Hello

Contract start date is 01.04.2016 and it is a Sales based contract.

We posted SBS for the month of April 2016

We then posted some on account payments without reference to the contract

We then try to post SBS for May 2016. system is showing some amount in payment previous period column. This column comes in txn RESRSE in simulation mode by clicking on results of sales based settlement.

Please explain what is this column all about and how does system calculate this amount?

Thanks

Rakesh