Period Billing or Partial Billing
If the client is set up for period billing in Media Accounting | Admin | Profiles Definition for period billing at the client or client/product or client/product/media or client/product/media/estimate profile level, then in the Buy Module, when you provide a Start and End Date for insertions for this client or client/product, client/product/media or client/product/media/estimate the multiple periods are calculated for you and will result in partial billings on draft and live bills.
If you want to change the number of periods to bill, and the billing profile is set up for period billing, in the Buy Module, you may key a different number of Periods to Bill for any insertion. The Previously Billed Increments column keeps you abreast of the billings status for this insertion.
If your company does not collect and release billing for a month or two, the next time you collect and release live bills, you may have billing for two or more periods.