Which data is included in the incremental backup? Does it contain last database and e-mail update or it's just contain last web data files?

Answer

Plesk includes the following data in incremental backups:
  • Web hosting data changed since the time of the last backup (full and incremental).
  • Mail data changed since the time of the last backup (full and incremental).
  • Full backup of databases.
So, incremental backup includes all newly created or updated information for web data (files and mail) and contains full databases copies. Information about the files (timestamps, size, permissons etc.) is stored in cid_user-data file. Example for Linux:
/var/lib/psa/dumps/domains/example.com/.discovered/backup_info_2104081233/cid_user-data
/var/lib/psa/dumps/clients/testcustomer/domains/example.com/.discovered/backup_info_2104010006_2104020000/cid_user-data