Delta File Transfer is off by default. However, you can enable it for a given backup selection.
See About Delta File Transfer.
In addition, if a maintenance server manages file servers that are on a target other than itself, the maintenance server must be configured for delegation.
See Configuring a maintenance server for delegation.
Delta File Transfer can also be selected as the default compression type. If the default compression setting is changed to Delta, all new backup selections use Delta compression by default.