When do we use “store period performance" function?

For example, this month is December, most of activity they update start and finish within December but there are always some activities which is updated in November or even in October. (They said they forget to update those activities in November so now they update it this month).

In this case P6 will add some more units to November and make your November report wrong. You have sent November report to other parties and it can not be edited. So, the only solution is you move all November "mistake" unit to December.

You can use Excel to save figure of November and December. Then we can have Actual this month by taking December minus November.

However, we can do this right in Primavera.

First in November you need to run Tool -> Store period performance, to storage Actual Units (in fact this is the cumulative actual unit) to November period.

In December you just update as normal. Now you can show the "Actual This Period Units" column. And that's the data we need.