A delta update produces a full baseline index, similar to the baseline update, but does so by joining a smaller extraction of source data (only the added, updated, or removed source records) with the output from the previous baseline update.
A delta update could be an option for you when you need to reduce the time required for loading the source data repository and for extracting the data.
For more information on delta updates and when to use them, see the solution article “Implementing Delta Updates” available online from the Endeca Developer Network (EDeN) at http://eden.endeca.com.