The _ets parameters allow to store the backup files from the SAP HANA Accelerator for SAP ASE option in a different path than the SAP HANA backup files.
They work the same way as the _es parameters for the Extended Storage option work.
The max_log_backup_group_size parameter is actually documented in the SAP HANA administration guide:
5.2.5.1.7.1.2 Log Backups and Third-Party Backup Tools
(wow, if that's not an impressive text locator, then I don't know what would... )