The Performance Indicator shows how the Campaign Item performs with current delivery data (depending on the Billing Category).
For currently running positions it calculates the performance for the future based on current Delivery Data, but always acts as if the current day is still giving Delivery Data.
This means that at the end of a day, when Delivery Data from that day came in, the Performance Indicator will be a bit higher.
When the item ends on the current day, there will be no further calculation and the field simply shows how much percent of the booked Quantity was delivered.