This document discusses how to create custom indices in Solr to index additional fields beyond what is supported out of the box. It explains that custom indices allow indexing fields that are not normally indexed, such as file attachments, and fields with data types that are not supported by facet display widgets. It provides examples of naming conventions for custom indices and code samples for implementing hook_apachesolr_index_document_build() to add custom fields to the Solr document and hook_apachesolr_query_alter() to make those fields available in search results. Reindexing is required after adding custom code. The new facets can then be configured and displayed.