The TagUsage.tsv file is essentially the same as the tag index pages displayed in Hyperview. It allows you to import a tagname, plus the definition of that tag in each of the databases or tag lists you have built. Tags that are listed in the Non-block points index are not included in TagUsage.tsv. It can be found in the EXPORTS folder of your project directory.
For more information, see the article Tag Index in Hyperview Help.