Database Tiers are customized by your institution's administrator in EBSCOadmin and determine which results will appear on your Integrated Search Result List.
Tier 1 Databases - When a database is designated as a Tier 1 database, results from that database will be displayed in the Result List when a search is executed.
Tier 2 Databases - When a database is designated as a Tier 2 database, searches will be executed in that database but results will not appear in the Result List. Results from Tier 2 databases are available by clicking on the hyperlinked database name in the Integrated Search Panel.
For more information, see: EBSCOhost Integrated Search - Configuration Best Practices