On this tab, you can specify the assembly and class that contain the implementation of your custom search index and fill in the Index data field.
•Assembly name - name of the assembly where the search index class is defined.
•Class name - name of the appropriate search index class.
•Index data - this field can be accessed from your code, so it can be used as a parameter to easily modify the index without having to edit its implementation.
Please note: You need to rebuild the index in order for changes made on this tab to take effect.
More resources can be found at: