Additional Data Sources
This guide explains how data sources other than OpenStreetMap mentioned in the install instructions got obtained and converted.
Nominatim uses pre-generated country borders data. In case one imports only a subset of a country. And to assign each place a partition. Nominatim database tables are split into partitions for performance.
More details in osm-search/country-grid-data.
US Census TIGER
For the United States you can choose to import additonal street-level data. The data isn't mixed into OSM data but queried as fallback when no OSM result can be found.
More details in osm-search/TIGER-data.
For Great Britain you can choose to import Royalmail postcode centroids.
More details in osm-search/gb-postcode-data.
Wikipedia & Wikidata rankings
Nominatim can import "importance" data of place names. This greatly improves ranking of results.
More details in osm-search/wikipedia-wikidata.