What is the difference between
Plesk uses access log files for the statistics calculation. The log files are modified by internal utilities during this process for statistics calculation purposes.
This is the log file using to keep current visits. It contains current actual state of a program (Apache in the case of
access_log) that writes the status and results of its work into such log files.
access_log is used to calculate statistics.
During the statistics calculation original
access_log files are analyzed by internal utilities. The extension .processed indicates that this log file has been already analyzed by statistics utilities.
This means that the file is compressed into gunzip archive depending on the Log rotation settings in order to save disk space. Log rotation settings could be changed on Service Plan level at Home > Service Plans > Default domain > Logs & Statistics tab.
For more details regarding log files refer to the Plesk Documentation.