Index Builder saves the indexes it produces in a tagged text file format. This allows Index Builder to embed styling information (Style Tags) in the indexes that it builds for you. Then, when you place the built indexes into a document where the specified styles have been set up, the embedded styling information will be automatically applied to the imported content.

To include style tags in a built index, use the Style Settings area of the Index Builder Preferences dialog to associate either InDesign Tags or MPS Tags to the different levels or types of an index. For example, you can enter a style tag to apply a paragraph style. Additionally, you can enter more than one tag for any level of the index that will be built. By default, the MPS Tag for one or more Tab characters 
("<\#9>") is present in a few boxes, as shown below:

Note you can also indicate whether you wish to produce indexes in the InDesign tagged text file format or the MPS tagged text file format. Choosing to Translate MPS Tags to InDesign Tags allows you to embed either or both MPS Tags and InDesign Tags in a built index. Additionally, you can indicate how to separate page numbers when more than one page number is assigned to a single index entry. 

For more information about InDesign Tags, please see InDesign documentation.

For more information about MPS Tags, please see the How to use MPS Tags to style variable data knowledgebase article.

For more details about Index Builder, please see the Index Builder manual, which is included in DesignMerge Catalog Documentation.