Reduced Delta is a feature that was introduced to allow you to optimize the network traffic for changes sent back to the server from the client.
By default, posting a change to the server will send the entire record, including all changed and unchanged fields. In contrast, reduced delta packets will only contain fields that are either part of the Primary Key (needed to locate the record that will be updated), and/or the fields that have actually been changed. This can greatly reduce network traffic, especially if your tables contain large blob fields that seldom change, or if you have very wide tables where usually only a small subset of the fields change.
As a downside, reduced deltas require that unique UPDATE statements are crafted by Data Abstract for each change and each delta may contain a different set of fields. When applying many changes at the same time, those updates won't benefit from having a single statement prepared on the back-end database to run consecutive times. Also, for the same reasons given for Dynamic Select, reduced deltas cannot be applied to data tables that provide their own commands for Inserts and Updates.
You can enable this feature by setting the SendReducedDelta property on your client-side streamer component to True. When deciding whether to leverage this new functionality or not, you should balance the gain from reduced network traffic (which largely depends on the structure and content of your tables) against the possible performance loss of not reusing prepared statements on the server (which in most cases should be negligible for all but large bulk updates).