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