Planning Using Primavera Project Planner P3 Version 3.1 Revised 2006

Autocost rules are the single most important aspect of P3 to be understood when statusing projects with costs and resources. Individually each rule is simple, but as a group they become quite complex and it is recommended that you experiment with a small schedule to confirm that your options are giving you the result you expect before you attempt a large project.
When this box is checked the percent complete is calculated as a proportion of Remaining Duration/Original Duration and visa versa. When either the Percent Complete or Remaining Duration is entered, then the other is calculated.
It is recommended that this is unchecked so you are able to enter the Percent Complete and Remaining Duration separately.
It this box is checked and you set the Remaining Duration to be greater than the Original Duration the Percent Complete will be set to Zero.
Checking this option results in an increase in Quantities and Hours to complete as the Remaining Duration is increased, and a decrease when the Remaining Duration is decreased. If this option is not checked then the Quantity to Complete remains constant and the Units per hour increase when the Remaining Duration decreases, and Units per hour decrease when the Remaining Duration increases.
It is recommended that this box is left unchecked so...