A payment line is a breakdown of your rebate earnings per program line by payment frequency. The Finance App displays payment lines which you can allocate cash to. The number of payment lines that are generated is based on the payment frequency of the program line. For example, a program line that spans a calendar year with a payment frequency of monthly will have 12 payment lines; one for each month. Similarly, a program line that spans a calendar year with a payment frequency of quarterly will have 4 payment lines; one for each quarter.
The number of payment lines that are generated is also based on the payment granularity of the program line. You can generate more granular payment lines by changing the Payment Granularity setting on the program line edit page. By choosing the payment granularity to be a dimension or an attribute you will create payment lines per dimension/attribute.