From the Server Configuration guide:
"By default, process data is purged automatically when disk space is required. If this parameter value is set, process data is unmounted or deleted, starting from the earliest date, until the size of the process store is less than the value.
This determines how big the total event store can get in MB before warm partitions are purged. When the threshold is reached, the oldest partition is either unmounted (converted to cold) or deleted from disk, based on value of AlwaysDeleteColdPartitions."