am a new bee to Duplicati , Got around 2.5 TB of data on file server with different department folder .
i can do the fullback without any issue , however how can i do the differential and incremental after taking a full backup .
Duplicati is a type of “incremental forever” backup. Subsequent backups will only back up changed blocks in new/changed files. When restoring, each backup version will be presented as a “full” backup.
Welcome to the forum!
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Great - many thanks 
Is there documentation on how this works?
Welcome to the forum @Tolli
The backup process explained is from user manual Introduction.
How the backup process works, section Processing similar data.
Block-based storage engine plus Whitepaper at end cover more.
Forum Developer category has some topics if you prefer deeper.