On this tab, you can see a list of custom tables that are included in the currently edited index. Custom tables can be added to the index using the Add custom table link. You can also Edit () the way listed custom tables are indexed or Delete () them from the list.
More resources can be found at:
Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?searchindex_customtable_index.htm