There is a lot going on in the Calculation tab of the Options dialog. It covers a large number of the default actions that Project takes when Project calculates the schedule for your projects. This task explains how the application-specific calculation options and the early/late task adjustments work.
Setting the Application Calculation Options
Automatic Calculation is the best Calculation mode for almost all situations. It ensures that the schedule, costs, and other calculations in your projects are always up to date. Use the Manual option ONLY if you have experience dealing with the outcome, as it can cause confusion.
If you decide to set the calculation mode to Manual, make sure you come back to this dialog often and click the Calculate Now button.
The Calculate radio buttons set the scope of the calculation when it occurs. The default is to calculate all projects that are currently open .
Setting Early/Late Task Adjustment Options
This option deals with tasks that started early, and tells Project to move the end of the finished portion of in-progress tasks that occur after the status date back to the status date.
This option is only available if the previous box in step 1 is checked, and also moves the start of the unfinished portion of the task back to the status date.
This option deals with tasks that started late, and tells Project to move the start of the remaining portion of in-progress tasks forward to the status date.
This option is only available if the previous box in step 3 is checked, and also moves the end of the completed parts of the task forward to the status date.