Doubt regarding tally receipt voucher

Silver (Student Others) (24 Points)

11 October 2014  
Hi, When we are entering a receipt voucher in Tally, there are 4 options to choose from: 1. On Account 2. Against Ref 3. New Ref 4. Advance Now, I understand Against Ref, New Ref & Advance but there is a slight confusion regarding on Account. What exactly does On Account mean?