Indexing creation and backup operations generate substantial amounts of indexing data. When this data is no longer required, the active index, index backups, action logs, and action log backups undergo a series of cleanup and compaction processes.
Note
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In Feature Release 11.18 and earlier, all indexing was performed at the backupset level. 
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In Feature Release 11.19, the Enable Subclient Index Workflow was introduced, allowing you to convert existing backupset indexing to subclient indexing on a client or client group. 
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In Feature Release 11.21 and later, subclient indexing is enabled for new file system agents by default.